The invention discloses a laser complex amplitude measuring method and system based on coherent diffraction imaging. The measuring method comprises the following steps: establishing an optical measuring system, wherein the optical measuring system comprises a laser, an optical attenuation group, an attenuator, a camera and a motorized translation stage; controlling the camera to move to a positionz1 by the motorized translation stage, and acquiring a D1 surface light spot image of the position z1 by the camera so as to acquire a first light spot image; controlling the camera to move to a position z2 at which the light spot size change can be seen clearly by the motorized translation stage, and acquiring a D2 surface light spot image of the position z2 by the camera so as to acquire a second light spot image; calculating an actual wavefront inclination of the laser; calculating complex amplitude of the first light spot image according to the actual wavefront inclination, an objective function and the second light spot image. The measuring method disclosed by the invention is high in sampling resolution and wide in dynamic range, and the laser complex amplitude can be accurately measured.