In ecologically sensitive and geographically constrained plateau regions, the dynamic relationship between cities and lakes constitutes a fundamental driver of regional sustainability. These landscapes exist in a state of deep interdependence, where the symbiotic interfaces - the transitional zones where urban and lacustrine systems converge-serve as vital arenas for continuous material and energy flows, as well as profound land use and land cover changes. The spatial configuration and functional management of these interfaces are therefore of paramount importance, directly influencing urban ecological resilience, biodiversity, microclimate regulation, and ultimately, the quality of human habitation. Focusing on the emblematic plateau lake-city system of Kunming and Dianchi Lake, this research employs a longitudinal, spatially-explicit analytical framework to unravel the complex co-evolution of urban development and lake ecology. By integrating multi-source remote sensing imagery, geographic information system (GIS) techniques, and statistical data spanning from 1990 to 2020, we systematically quantified the spatiotemporal patterns of urban expansion and lakeshore transformation. The core of our analysis lies in applying a coupled coordination degree model to evaluate the synergistic symbiosis level of the city-lake landscape system, moving beyond descriptive analysis to a quantified assessment of system harmony. The principal findings of this study reveal a nuanced trajectory of interaction and co-adaptation: 1) Spatial-Temporal Dynamics: The evolving tension and negotiation between urban spatial growth and the imperative for lake ecological protection have been the dominant force reshaping the landscape symbiotic interfaces. This has manifested in cyclical patterns of encroachment, retreat, and stabilization, fundamentally altering the structure and functionality of these critical edge zones. 2) Evolution of Synergy: The city-lake landscape relationship has undergone a significant qualitative shift, transitioning from a prolonged period of disharmony and maladjustment (characterized by urban sprawl at the expense of lake health) to an emerging phase of preliminary coordination and mutualism in recent years. Nevertheless, this coordinated state remains fragile and nascent, with the calculated coordination index indicating substantial scope for progression towards a more robust and stable symbiotic regime. 3) Multifaceted Drivers: The pathway towards synergistic development is not deterministic but is mediated by a complex, hierarchical set of interacting factors. Our analysis identifies a confluence of drivers including macro-level economic and demographic policies, the immutable constraints and opportunities presented by the natural terrain and hydrological environment, and the tangible impacts of specific urban planning decisions and construction activities. In conclusion, fostering ecologically livable cities in fragile plateau lake regions requires a holistic, system-level governance approach. This study posits that future strategies must pivot towards two synergistic core pillars: firstly, the intentional enhancement of ecosystem service quality through ecological restoration, blue-green infrastructure networks, and sustainable land use planning within the symbiotic interfaces; and secondly, the implementation of integrated, watershed-scale governance mechanisms that transcend administrative boundaries. Such mechanisms should coordinate water resource management, pollution control, spatial planning, and socio-economic development to align human activities with the carrying capacity of the lake ecosystem, thereby securing a sustainable and synergistic future for plateau city-lake landscapes. Notwithstanding these contributions and policy implications, this study acknowledges certain limitations. The construction of assessment indicators for the city-lake system was constrained by the availability and continuity of spatiotemporal data. Furthermore, the underlying mechanisms driving city-lake symbiosis were primarily explored through qualitative analysis, warranting future quantification and mechanistic modeling. Looking ahead, we propose that future research adopt a more systematic and in-depth perspective to explore the synergistic co-evolution between the entire Dianchi water system-encompassing its tributaries, catchment, and hydrological cycles - and the urban fabric, which would further illuminate pathways toward sustainable coexistence in complex plateau lake basins.