The invention discloses a method for detecting the plane residual stress of a thin plate after laser shot peening, and the method comprises the steps: transmitting a cutting instruction into a cuttingdevice, so that the cutting device cuts a rectangular sample plate from the thin plate according to the cutting instruction; fixing the sample plate on a support bottom plate which passes through a groove in the thickness direction, and controlling an immersion-type electric spark cutting machine tool to pass through the groove of the support bottom plate to cut the sample plate to obtain a firstcutting surface and a second cutting surface; and detecting the plane residual stress of the first cutting surface and the second cutting surface to obtain the plane residual stress state of the thinplate. The method can avoid the deformation of the cutting surface caused by the action of gravity, and improves the detection precision of the plane residual stress of the thin plate.