A method for preparing polymethylene polyphenyl polycarbamate is provided. The method includes dissolving phenylcarbamate in a water-immiscible organic solvent to form a solution A, formulating an aqueous acid catalyst solution to form a solution B, forming a reaction system comprising an organic phase and an aqueous phase by firstly adding a methylating reagent to the solution B and then mixing the solution A and the solution B, wherein the organic phase comprises phenylcarbamate and the aqueous phase comprises the acid catalyst and the methylating reagent. The method further includes reacting the reaction system under stirring at a reaction temperature of 30° C. to 200° C. and a reaction pressure of 0.05 MPa to 5 MPa to produce a polymethylene polyphenyl polycarbamate product mixture dissolved in the organic phase, and separating the polymethylene polyphenyl polycarbamate product mixture by allowing the organic phase and the aqueous phase to stand still and stratify.