The invention relates to a polyformaldehyde dimethyl ether synthesis method which mainly solves the problems of difficult catalyst separation, low raw material conversion rate and poor product selectivity in the prior art. The technical scheme of the polyformaldehyde dimethyl ether synthesis method comprises the following steps: by taking methanol, methylal and polyformaldehyde as raw materials, under the conditions that the reaction temperature is 70-200 DEG C and the reaction pressure is 0.2-6 MPa, enabling the raw materials to be in contact with a catalyst, and reacting to generate polyformaldehyde dimethyl ether, wherein the weight ratio of the methanol, the methylal and the polyformaldehyde is (0-10):(0-10):1, and the amounts of the methanol and the methylal can not be 0 at the same time; the amount of the catalyst accounts for 0.1-5.0% of the weight of the raw materials; and the used catalyst comprises the following components in parts by weight: a) 30-80 parts of carrier, which is at least one selected from SBA-15, MCM-41 or MCM-22 molecular sieve, and b) 20-70 parts of solid super-strong acid carried thereon, which is at least one selected from SO4<2->/ZrO2, SO4<2->/Fe2O3, Cl<->/TiO2 or Cl<->/Fe2O3. Thus, the problems are well solved; and the polyformaldehyde dimethyl ether synthesis method can be used for industrial production of polyformaldehyde dimethyl ether.