A method of a polyurethane foam includes the following steps of: (1) simultaneously pumping a mixed solution prepared from hydrogen peroxide, an organic acid, a catalyst and a stabilizer and a vegetable oil into a first microstructured reactor of a micro-channel modular reaction device for reacting to obtain a reaction solution containing epoxidized vegetable oil; (2) simultaneously pumping the reaction solution containing the epoxidized vegetable oil obtained from the step (1) and a compound of formula III into a second microstructured reactor of the micro-channel modular reaction device for reaction to obtain a vegetable oil polyol; and (3) reacting the vegetable oil polyol prepared from the step (2) with a foam stabilizer, a cyclohexylamine, an isocyanate and a foaming agent cyclopentane for foaming so as to prepare a rigid polyurethane foam.