When Class II reservoirs entered the late stage of polymer flooding, the injectivity decline and injection difficulties appeared. To solve these problems, this study focused on four regions of Block X as the research subject. The wells were categorized into four types, and three influencing factors of the injectivity decline were analyzed: single sand body thickness, permeability, injection system and water quality. The results indicated that reservoir development characteristics and well-group continuity were the main controlling factors for consistently poor injectivity during polymer flooding stage. However, for the gradual or intermittent decline in injectivity during the late stage of polymer flooding, areal heterogeneity of the oil reservoir was the fundamental factor, while changes in the injection system and water quality fluctuations were the key contributing factors. Through specific methods such as chemical blockage removal, fracturing, fracturing combined with chemical unblocking, optimization of injection parameters, periodic polymer flooding and stopping polymer injection. A comprehensive methodology was developed to solve the injection problems of different well types in different regions for Class II reservoirs during the late stage of polymer flooding. These methods significantly enhanced injectivity and improved injection effect. Therefore, this paper also provides actionable insights and reference suggestions for addressing similar issues during late-stage development of Class II reservoirs in the future.