The invention discloses a synthesizing method of trioxymethylene. The method comprises the following step that: formalin is reacted under the catalysis of ionic liquid, and then the trioxymethylene is obtained, wherein the percentage by weight of the formalin is 30-70%. A catalyst used in the method disclosed by the invention is easy to prepare, and has high catalytic activity, the concentration of the trioxymethylene in reaction liquid is higher, the contents of methanal, methanoic acid, methylal and other by-products are low, and the selectivity of a product is better; the application scope of the concentration of the methanal material is wide and is 30-70 wt%, and the reaction condition is mild; the dosage of the ionic liquid catalyst is small and is 0.05-10 wt%; and the catalyst has good heat stability and can be recycled.