To investigate the complex wake flow characteristics of multi-motor parallel rocket sleds, this study focuses on the mechanisms by which the nozzle horizontal spacing and the impingement height influence the flow structure and ground effect. A three-dimensional physical model was constructed for a dual-rail rocket sled system featuring three nozzles arranged in a triangular pyramid configuration. Four operating conditions were established, including large spacing (s=7d), small spacing (s=1d), low impingement height (h=2d), and high impingement height (h=5.5d). Here, s represents the horizontal center-to-center distance of the nozzles, h denotes the impact height, and d signifies the diameter of the nozzle outlet. The effects of nozzle center distance and impingement height on the flow field structure and ground effect were comparatively analyzed. Numerical simulations were performed using a computational fluid dynamics (CFD) method based on the Reynolds-Averaged Navier-Stokes equations, coupled with the Realizable k-ε turbulence model for transient solutions. The combustion chamber pressure-time curve derived from interior ballistic theory was applied to the nozzle inlet via a user-defined function (UDF). The sled velocity-time curve, determined from the exterior ballistic particle trajectory equation, was assigned as the far-field pressure boundary condition, enabling a coupled simulation framework of interior ballistics, exterior ballistics and flow field. The computational domain utilized a structured grid with refinement in the jet interaction region and near the ground to ensure calculation accuracy. The velocity and pressure fields obtained from numerical simulations were compared and validated against jet morphology, impingement height, and vortex core positions recorded by high-speed photography (2000 Hz). The flow field structure, pressure distribution, and thermal erosion behavior on the ground under different configurations are systematically revealed. The results indicate that the small-spacing nozzle arrangement triggers intense jet interference without ground effect participation, leading to a multi-peak and slow-recovery pressure evolution feature and substantially delays the flow field relaxation process. The coupling relationship between ground effect and jet interference is dominated by impingement height. At low impingement height conditions, the jet impinging on the ground induces intense reorganization and fragmentation of vortex structures, generating wall jets with velocities up to 960 m/s. Consequently, the peak ground surface temperatures reaches 1286.6 K with sustained high temperatures, which significantly elevates the risk of rail ablation. Conversely, a high impingement height effectively suppresses the ground effect, resulting in a more homogeneous and stable flow field structure. In this case, the peak ground temperature reduced by approximately 65% and maximum velocity reduced by 58%, significantly mitigating ablation risk. The initial phase (0－8 m) of the rocket sled is identified as the critical region subjected to the most severe thermomechanical loads. During this stage, the average acceleration reaches 832.7 m/s², and the specific action time per unit distance is prolonged to 1.84 ms/m. Coupled with the transient complex flow field, this constitutes an extremely high risk for rail ablation. The numerical simulation results show excellent agreement with high-speed photographic experimental data regarding flow field morphology, impingement height, and vortex core positions, thereby validating the reliability of the established coupled model. This study elucidates the complex flow mechanisms of multi-nozzle parallel systems under strongly constrained conditions, and provids important theoretical foundations and design parameters for structural layout optimization and thermal protection design in high-acceleration, heavy-load rocket sled test systems.