This paper describesthe optical-mechanical structure design and analysis of a Cassegrain reflection system (here in after referred to as Cassegrain subassembly or Cassegrain system). At first decompose the index according to the design input. Finish the design of the Cassegrain subassembly. Then iterate and verify the design by tolerance and simulation analysis. At last verify the reliability and environmental probability of the machined and adjusted Cassegrain subassembly by environmental tests. The Cassegrain subassembly described in this paper has characteristics of high stability and rigidity, strong reliability and environmental probability. Also it meets the optical performance requirements.