The invention discloses a device and a method for measuring the electricity of saved electric power before and after modification of a transformer, and belongs to the technical field of modification of transformers. The device comprises an active electricity acquiring module, a reactive electricity acquiring module, an average voltage acquiring module, an acquisition period setting module, a project period setting module, an average current computing module, a short-circuiting loss electricity computing module, a non-load loss electricity computing module and a saved electricity computing module. The method comprises the following steps of calculating active electricity of the transformer in a period; calculating reactive electricity of the transformer in the period; performing computing to obtain an average voltage; setting an acquiring period; setting a project period; performing computing to obtain an average current which flows through the transformer in the acquiring period; performing computing to obtain short-circuiting loss electricity of the transformer in the project period; performing computing to obtain non-load loss electricity of the transformer in the project period; and performing computing to obtain the saved electricity. By the device and the method, the electricity saved before and after modification of the transformer can be measured.