The solar blind ultraviolet optical detector accurately captures the ultraviolet radiation generated by corona discharge. In this paper, a fixed focal solar blind optical system was designed by using Zemax. The system comprises five standard spherical lens and has a working wavelength range of 240-280 nm, the F number of 2.5, the focal distance of 50 mm, a field angle of 20.4°, an optical length of 72 mm and matches with a ultraviolet image intensifier with a target diameter of 18 mm. The full field of modulation transfer function is greater than 0.8 at the spatial frequency of 20 lp/mm, the maximum distortion is less than 0.5％. The defocusing phenomenon of the system was analyzed across a temperature range of -20℃ to 60℃. The thermal defocusing amount was calculated at different temperatures, and a Passive mechanical compensation method was employed to correct thermal dispersion caused by temperature changes. Finally, through Monte Carlo simulation analysis, the system is given a reasonable tolerance allocation. The analysis results show that the system still maintains good imaging quality even after processing and assembly.