Abstract: In order to meet the demands of a microcosmic particle observing camera for its optical system, a unique focusing mechanism driven by a ball screw in the linear movement was presented and the stress and strain of the focusing mechanism were analyzed by the Finite Element method(FEM) in accordance with its temperature and the static loads. Results show that the biggest strain is 0.001 mm under the static load，and 0.002 4 mm under the temperature load. The accessory error and the assembly error were analyzed, it is shown that the total comparative error is 2.2%. In the specimen experiment, the resolving power of 0.005 mm at each step and the average backlash error of 0.001 mm are obtained, respectively, which meets the need of the system.