With the rapid development of modern technology, theodolite test device received a lot of attention.This paper introduces the structure of theodolite test device briefly. We designed a theodolite test device which could test dual-band theodolite, and designed the Cassegrain collimator. We utilized Zemax optical design tool to design and analyse the optical system. We determined the aspheric coefficient. In the analysis of the spot diagram, MTF function and energy distribution over the image quality, we determined the feasibility and rationality of the design. Finally based on the analysis,we designed mechanical structure.