The invention discloses a universal support device for an electrical module under a vehicle and belongs to the field of a pre-assembled positioning auxiliary support device for an electrical module under a rail vehicle. The universal support device comprises a plurality of main cross beam positioning support mechanisms, a plurality of right-angle connection sliding blocks and a basic frame. The basic frame is a rectangular frame. The upper end surfaces of two longitudinal x-axis long sides are provided with frame T-shaped chutes. The main cross beam positioning support mechanism comprises twosupport beams, two main cross beam width positioning bases, two main cross beam vertical height support base and a support beam transition clamping plate. The universal support device for an electrical module under a vehicle can be widely used for positioning and assembling of corresponding main beams and electrical modules under various types of vehicles, save manpower, material resources and a narrow space, reduce resource wastes and reduce a production cost.