A prefabricated stair modular assembly mold comprises a base, a step standard module, an upside module of a lower platform, an underside rotating module of a lower platform, an upside module of an upper platform, an underside rotating module of an upper platform, an underside common module, and an underside adjustment modules. Prefabricated stair molds of different dimensions are formed by means of locating holes provided in the assembling modules, and are then fixed by means of sliding holes at two ends. Further disclosed is a mounting method for said assembly mold. Said assembly mold has high positioning accuracy, and small accumulated error, improving the mounting efficiency of the prefabricated staircase mold.