In this study, a solar blind ultraviolet optical system with a fast focal ratio and long focal length is presented. Its F number is 2.0, and its focal length is 100 mm. The ratio of the total length to its focal length is approximately 1.5. The proposed system comprises all spherical lenses without using aspherical or diffractive optical elements, which is convenient for manufacturing and testing. The positive lenses are made of calcium fluoride, and the negative lenses are made of fused quartz, which are used to effectively correct the color aberration. The maximum lateral color aberration is less than one pixel in size. Design results show that the proposed system exhibits excellent imaging performance, high resolution, and is compatible with the characteristics of fast focal ratio and long focal length, which satisfies the design requirements. Furthermore, the tolerance analysis demonstrates that the proposed system can be realized in the practical engineering.