In order to realize the laser additive manufacturing of complex parts with unequal wall thickness, a variable circular spot laser cladding optical system based on adaptive mirror is proposed. The system consists of reflective parabolic collimating mirror, reflective adaptive mirror and reflective parabolic focusing mirror. The air pressure proportional valve is used to adjust the gas pressure in the adaptive mirror cavity, to change the radius of curvature of the adaptive mirror surface, and make the beam incident on the reflective parabolic focusing mirror at different convergence angles or divergence angles, so as to change the spot size acting on the surface of the workpiece. Physical model is established based on adaptive mirror variable spot optical system, and using Matlab and Zemax softwares, the effects of the range of the adaptive mirror focal length, the focal length and position of the reflective parabolic collimating mirror and the reflective parabolic focusing mirror, and the parameters of the fiber laser source on the variable spot characteristics of the system are analyzed. A 6 kW fiber laser is used to carry out cladding experiment according to the scheme of variable spot, variable power, constant speed and constant power density, and an elliptical workpiece with a wall thickness of 3-8 mm is realized.