The invention relates to a preparation method of a psychotropic medicine asenapine. The method comprises a step that asenapine is prepared through a reaction of a raw material trans-5-chloro-2,3,3a,12b-tetrahydro-2-methyl-1H-monobenzo[2,3:6,7]oxepino[4,5-C]pyrrole-1-one or trans-11-chloro-2,3,3a,12b-tetrahydro-2-methyl-1H-dibenzo[2,3:6,7]oxepino[4,5-C]pyrrole-1-one under the action of a reducing agent sodium bisaluminumhydride or a composite reducing agent composed of sodium bisaluminumhydride and other regents. The method has the advantages of mild and controllable reaction technology, low production cost, good product yield, realization of the large scale application in the industrial production, and good enforcement values and social and economic benefits.