The present invention relates to a method of synthesizing trioxymethylene from formaldehyde by the catalytic action of an acidic ionic liquid. In the method, formaldehyde solution with a concentration of 30.about.80 wt % is used as reactant, and an ionic liquid is used as catalyst. The cation moiety of the catalyst is selected from either imidazoles cation or pyridines cation, and the anion moiety of the catalyst is selected from one of p-tolyl benzene sulfonate, trifluoromethyl sulfonate, and hydrogen sulfate. In the present invention, ionic liquid is used, for the first time, as a catalyst to synthesize trioxymethylene from formaldehyde. The catalyst can be circularly used for continuous sampling.