In this paper, the structural design and assembly process of optical system with variable F-number switching mechanism are studied. The variable F-number design of the infrared optical system is achieved by alternately switching between two sets of optical lens groups. In assembly process, the reference optical lens group is positioned through mechanical frock, while the variable F-number lens assembly is adjusted based on the field of view (FOV) of the reference assembly. Both focal plane and optical axis alignments are achieved through respective adjustments, ensuring system-wide consistency in focal plane alignment and optical axis collinearity. The proposed switching mechanism features a compact structure, short switching stroke, rapid switching speed, and convenient assembly-alignment processes. It satisfies stringent requirements for optical axis stability and focal plane consistency.