The invention relates to a fixing device for a mainboard, which is used for fixing the mainboard provided with a plurality of positioning holes. The fixing device for the mainboard comprises a supporting stand, and the supporting stand comprises a first supporting surface. The first supporting surface is provided with a plurality of fixing parts which are locked in the corresponding positioning holes on the mainboard. The fixing parts of the first supporting surface are used for supporting the mainboard when the mainboard is vertically arranged. The fixing device for the mainboard also comprises a plurality of fixing rods; the supporting stand also comprises a second supporting surface; the second supporting surface is also provided with a plurality of fixing parts corresponding to the positioning holes on the mainboard. One end of the fixing rods is respectively locked at the corresponding fixing part of the second supporting surface, and the other end is respectively fixed in corresponding positioning hole of the mainboard when the mainboard is conversely arranged, thus the function of fixing the mainboard with two surfaces is achieved.