The invention relates to a method for preparing a FeSe superconducting thin film by post-selenization treatment;in the method, a FeSe thin film is deposited by reactive sputtering; the method comprises the following steps: performing surface treatment of a substrate; sputtering and depositing a FeSe thin film on the substrate by reactive sputtering film deposition technology; putting both the FeSe thin film and amorphous simple substance selenium in a quartz tube, performing vacuum sealing; putting the sealed quartz tube in a heating furnace, heating to perform a selenization reaction to obtain the FeSe thin film. With the invention, the stress generated after selenization treatment is significantly reduced; impurity phases, air pores and cracks generated as a result of excessive distortion energy are reduced; and excellent superconductive performance of the thin film is brought into play. Additionally, when compared with commonly-used pulse laser deposition methods, the preparation method combining reactive sputtering with improved selenization treatment has great advantages in cost control, and is not restricted by dimensions.