The invention provides grafted polyoxymethylene and a preparation method thereof. The grafted polyoxymethylene is prepared by reactively extruding raw materials including polyoxymethylene resin, an initiator, an active monomer, an antioxidant and a formaldehyde absorbent; polyoxymethylene is prepared by performing broken chain type reaction on the raw materials. According to the grafted polyoxymethylene and the preparation method, the grafting of polyoxymethylene can be realized, the grafted chain segments have active reaction functional groups, polyoxymethylene has strong polar group, and the modified composite compatibility and performance promotion of polyoxymethylene can be realized; the defect that polyoxymethylene lacks a good compatilizer can be overcome, and the mechanical performance and application field of polyoxymethylene can be increased. In addition, the preparation method is simple in technology, strong in operability, high in production efficiency and low in cost.