In the later stage of thermal composite recovery of heavy oil，although particle plugging and adjustment operations have obvious effects on further enhancing oil recovery，the influence of the thermal composite fluid remaining in the reservoir on the settling characteristics of plugging agent particles is still unclear，which limits the application of particle suspension systems in this stage. Therefore，based on the particle settling velocity theory，the basic physical properties of the particle suspension system and the inherent laws of the particle settling velocity are studied. Through the particle suspension experiment and Zeta potential experiment，the settling characteristics and mechanism of suspended particles under different factors are analyzed. The results show that a larger ratio of the particle size to the pore diameter indicates greater particle settling velocity；when pH = 5 or pH = 7，the stability of the particle suspension system is better；when pH = 9，the particle suspension system produces obvious coagulation and sedimentation phenomenon；anionic or nonionic surfactants have little effect on the stability of the particle suspension system，while cationic surfactants and particle suspension system produce flocculation. However，as the pH value decreases，the flocculation weakens，and when the pH value is 3，no flocculation occurs. Therefore，choosing a suitable particle suspension system is of great significance to the success or failure of deep plugging and adjustment operations in the later stage of thermal composite recovery of heavy oil.