The invention relates to a preparation method of polyformaldehyde copolymerized and modified by an oxetane derivative. The preparation method mainly comprises the following steps of: adding the oxetane derivative, trioxymethylene and an initiator in a reaction flask, and preparing copolyoxymethylene at certain temperature in a body polymerization or solution polymerization manner. The preparation method has the following characteristics: 1) the synthesis method is simple and efficient, raw materials and reagents adopted in the reaction process are cheap and liable to obtain, the operation process is liable to amplify and industrial production is easy to realize; 2) the oxetane with two side chains is copolymerized with the trioxymethylene for the first time to introduce 3-50% of oxetane structure with side chains onto a main polymerization chain to change the molecular structure of the polyformaldehyde from the essence so as to affect the crystallization property of the polyformaldehyde molecular chain, greatly perfect the degree of crystallinity of the polyformaldehyde and greatly adjust the rigidity and thermal stability of the polymer. The preparation method is possibly used for converting the polyformaldehyde into low-cost common plastics, thereby having active and important significance of developing new application markets of polyformaldehyde.