In the product structure design, along the existing products or by virtue of the existing experience is a common phenomenon, infrared thermal imaging camera continuous zoom mechanism optical axis jump overrun has been the industry's problem, and the size of its jump directly affects the system's performance indicators. In this paper, Taking the guide rod two-component linkage continuous zoom mechanism as an example, the factors affecting the runout of the optical axis of the continuous zoom mechanism are firstly analyzed from the perspective of assembly, and then a clear, reasonable and correct assembly dimension chain model is constructed based on the principle of dimension chain by analyzing the associated dimensions. Then, through the transformation of optical machine parameters, the tolerance of the closed ring in the dimensional chain is determined. Finally, based on the principle of tolerance design, the tolerance of each component ring in the dimensional chain is assigned, and the limit deviation of the closed ring is accounted for by means of tolerance analysis, so as to ensure that the tolerance design of each component ring is correct and error-free.