The invention provides an expansion device of a wind turbine generator system. The expansion device comprises a tower and a first fixed pile; and the tower is connected with the first fixed pile. Theexpansion device is characterized by further comprising a truss tower, a connecting piece, an anchor bolt, a second fixed pile, a flange, a first tower and an expanded wind turbine generator system; the truss tower is connected with the tower through the connecting piece; the tower is positioned in the truss tower; the bottom of the truss tower is fixedly connected with the second fixed pile through the anchor bolt; the flange is arranged at the top of the truss tower; the first tower is arranged on the flange; and the expanded wind turbine generator system is arranged on the first tower. Theexpansion device solves the problems of low efficiency and high cost in expansion of a traditional wind turbine generator system.