This study aimed to explore the effects of laminarin (LM) on the physicochemical properties of sweet potato starch (SPS) and pea starch (PS), and to reveal the interaction rules between LM and the two starches by measuring the interaction forces in the starch-LM mixed systems. Different concentrations of LM (0%, 1%, 2%, and 3%, w/v) were mixed with the two starches respectively. The changes in gelatinization and rheological properties were measured using a rapid viscosity analyzer and a rheometer, and the gel strength and interaction forces were analyzed using a texture analyzer. The results showed that LM could inhibit the leaching of amylose from SPS, while increasing the leached amylose content of PS. In terms of gelatinization properties, the addition of LM increased the gelatinization viscosity of SPS-LM and PS-LM mixed systems, while reducing the setback viscosity of SPS and increasing that of PS. In rheological properties, the addition of LM increased the fluid index (n value) of SPS, but for PS, the addition of high concentrations (2% and 3%) of LM could reduce its fluid index (n value). The addition of LM enhanced the resilience of SPS and PS, and significantly reduced the thixotropic loop area of both. The addition of LM increased the storage modulus (G') and loss modulus (G") of SPS and PS. LM had different effects on the physicochemical properties of the two different starches, which might be related to the interaction mode between LM and the starches. The interaction forces indicated that hydrogen bond forces were mainly present in the SPS-LM mixed system, while both hydrogen bond forces and electrostatic forces were present in the PS-LM mixed system. In conclusion, LM interacted with SPS via hydrogen bonding during gelatinization, significantly elevating viscosity and viscoelasticity. In contrast, PS-LM interactions involved both hydrogen bonds and electrostatic forces, synergistically enhancing pseudoplasticity, gel strength, and rheological properties (including setback value). This study provides a theoretical basis for the development of laminarin-starch products.