In order to realize laser additive manufacturing and damage repair of complex parts with different sizes, an adjustable variable spot size laser cladding optical system is proposed. The system is an off-axis three-mirror optical system, which is composed of three aspherical reflectors. It can adjust the spot size by moving an asperical mirror with an step motor. It can eliminatesolves the problem of light energy loss fixed spot size of traditional refractive optical systems. The proposed solution is high efficient in complex parts clading process and cost effective. and lift the application limitation of high-power lasers in laser cladding. In this paper, the physical model of off-axis three-mirror optical system is established by means of Zemax software. A 4000 W fiber laser in non-sequence mode was used to simulate the intensity distribution of the spot cross-section. By defocusing, the spot diameter at the working face can change continuously from 6mm to 12mm.