To meet the needs of metal processing with different materials and different thicknesses, we design a variable spot and zoom laser cutting optical system which consists of a composite collimating lens group, a zoom lens group, a compensated lens group, and a focusing lens group. We adopt an adjustable magnification expansion beam system in the middle of the collimating lens group and the focusing lens group of the ordinary optical focusing system, and then combine the collimating lens group and the adjustable magnification beam expanding system into a composite collimating lens group to simplify the system. We establish a physical model of the variable spot and zoom optical system with four lenses, derive the geometric locus of zoom group and compensated group. Theory verification is performed with MATLAB software. Based on the existing cutting optical system, we design a variable spot and zoom laser cutting optical system, and optimize the aberrations by using software. The results show that when we move the zoom lens group and the compensated lens group, the magnification is from 1.000 to 3.750, and the focus up and down adjustable range is from -20 mm to +10 mm. The precise adjustment of the focal spot size and focus position is thus realized.