Disclosed by the present invention are a polyformaldehyde thin film and a preparation method therefor, the polyformaldehyde thin film comprising the following components in parts by weight : 96-98 parts of a polyformaldehyde resin, 0.5-2 parts of a nucleating agent, 0.5-1 parts of an antioxidant, and 0.5-1 parts of a formaldehyde absorbent. For the characteristics of high crystallization rate and fast crystallization speed of polyformaldehyde, a film blowing method is used to directly blow a melt extruded from a die of an extruder to form a film, thus overcoming the problem wherein necking and cracking easily occur during a stretching process, being beneficial in polyformaldehyde film forming, and having high processing efficiency and being easy to industrialize; a copolymerization unit -CH2-CH2-O- is introduced into the molecular chain of polyformaldehyde, and the proportion thereof is increased, which may effectively reduce the crystallization rate of a material such that same may easily form a film; a multi-stage air-cooled cooling mode is used to control the crystallization speed of polyformaldehyde and prevent local high crystallization; when the foregoing is used in combination with an application of a nucleating agent, spherulite size is reduced, crystal regularity and packing compactness are increased, inter-molecular force is increased, and the prepared polyformaldehyde film has excellent surface smoothness.