This invention describes a method for preparing polymethoxymethylal. By this method, polymethoxymethylal is prepared by a catalytic reaction using methanol and trioxymethylene as reactants and using an ionic liquid as catalyst under a relatively moderate reaction condition. The catalyst of this invention has a high catalytic activity and a high conversion; the reaction process is simple, easy to be operated and has a strong controllability; the distribution of the products after reaction is superior and the utilization ratio of the raw materials is high.