Combined with SEM, EDS, dynamic evaluation of plug formation, microscopic visualization and long core displacement experiments, the damage mechanism of residual acid in polymer injection oilfield to the reservoir was studied. The results show that the damage of residual acid to the reservoir is mainly due to the crosslinking point of the polymer molecular chain with high valence cations such as Al³⁺ and Fe³⁺ in residual acid. The crosslinking network elastic colloid is formed by the coordination between carboxylic acid radicals in the polymer side group and high valence cations, and the degree of crosslinking increases with the increase of ion concentration. The elastic colloid formed blocked at the edge and pit of the pore throat, which makes the pore throat diameter gradually smaller. In the process of flow, it wraps around the formation particles and carbonate sediments, forming a composite blockage micelle, resulting in deep plugging. The long core displacement experiment shows that residual acid can damage the core up to 60%. The research results provide a theoretical basis for the research and development of plugging removal fluid system and optimization of plug removal processes in polymer injection oilfield