Salt-tolerant modified starch (SMS) is a novel green chemical agent used to reutilize the residual partially hydrolyzed polyacrylamide (HPAM) in offshore reservoirs for enhanced oil recovery. In this study, laboratory experiments are conducted to investigate the mechanism of flocculation and conglomeration between the synthetic SMS and HPAM at different concentrations. The experimental results show that SMS reacts with the residual HPAM to form a floc by charge neutralization and adsorption bridging mechanisms when the concentration of residual HPAM is low. With the increase of the residual HPAM concentration, SMS blends with the HPAM to form a gel-like conglomeration (GLC) with high viscosity and strength. SEM and ESEM images of the microstructures of the floc and GLC show irregular multilayer network structures. Enhanced oil recovery tests in parallel-core models prove that the injection of an SMS slug after HPAM flooding can effectively improve the sweep efficiency and residual oil in the low-permeability zone. Laboratory core tests provide credible proofs for the large-scale application of this polymer reutilization system in offshore oilfields.