Abstract: The pilot test of infilling polymer-surfactant-preformed particle gel (PPG) flooding has been successfully implemented after polymer flooding in Ng3 block of Gudao Oilfield in China. However, the production characteristics and displacement mechanisms are still unclear, which restricts its further popularization and application. Aiming at this problem, this paper firstly analyzes the production performance of the pilot test and proposed four response types according to the change of water cut curves, including W-type, U-type, V-type response, and no response. Furthermore, the underlying reasons of these four types are analyzed from the aspects of seepage resistance and sweep efficiency. The overall sweep efficiency of gradual-rising W-type, gradual-decreasing W-type, and early V-type response increases from 0.81 to 0.93, 0.55 to 0.89, and 0.94 to 1, respectively. And the sum of seepage resistance along the connection line between production well and injection well for U-type and delayed V-type response increases from 0.0994 to 0.2425, and 0.0677 to 0.1654, respectively. Then, the remaining oil distribution after polymer flooding is summarized into four types on the basis of production and geological characteristics, namely disconnected remaining oil, streamline unswept remaining oil, rhythm remaining oil, and interlayer-controlled remaining oil. Furthermore, the main displacement mechanisms for each type are clarified based on the dimensionless seepage resistance and water absorption profile. Generally, improving connectivity by well pattern infilling is the most important for producing disconnected remaining oil. The synergistic effect of well pattern infilling and polymer-surfactant-PPG flooding increases the dimensionless seepage resistance of water channeling regions and forces the subsequent injected water to turn to regions with streamline unswept remaining oil. The improvement of the water absorption profile by polymer-surfactant-PPG flooding and separated layer water injection contributes to displacing rhythm remaining oil and interlayer-controlled remaining oil. Finally, the paper analyzes the relationships between the remaining oil distribution after polymer flooding and production characteristics of infilling polymer-surfactant-PPG flooding. The study helps to deepen the understanding of infilling polymer-surfactant-PPG flooding and has reference significance for more commercial implementations in the future.