The invention belongs to the technical field of pharmaceutical and chemical industry, and particularly relates to a method for refining a trimethoprim cyclizing process recycled product. The method comprises the following steps: performing liquid-phase analysis on a trimethoprim cyclizing process recycled product to detect the content of 4-demethylated trimethoprim; adding the trimethoprim cyclizing process recycled product into methanol and caustic soda flakes under a stirring state, heating and refluxing to dissolve, concentrating methanol to dry; refluxing by adding water, cooling to room temperature, and filtering to obtain trimethoprim; regulating the pH value of filtrated mother liquor to be 8 with acetic acid, separating crystal, and filtering to obtain the 4-demethylated trimethoprim. The method is scientific, reasonable, simple and feasible, and can be used for reducing cost and saving raw materials.