The invention relates to a preparation method of polymethoxymethylal, mainly solving the problem that in the prior art, the catalyst has corrosivity. The method comprises the following step: by taking methanol or dimethyl ether and formaldehyde or trioxymethylene as raw materials, contacting the reaction raw materials with a catalyst to generate the polymethoxymethylal under the conditions that the molar ratio of the methanol or the dimethyl ether to the formaldehyde or the trioxymethylene is 1: (0.1-10), the reaction temperature is 50 to 200 DEGC and the reaction pressure of 0.1-10 MPa, wherein a molecular sieve ZSM-5 with the size of less than or equal to 5 microns is adopted as the active component of the used catalyst. through using the technical scheme, the problem is solved well. The method provided by the invention can be applied to industrial production of the poly methoxy methylal.