The changes in ambient temperature will cause thermal focus shift in the optical system, which leads to the instability of the image quality. Due to the limitation of materials, the athermal design process and results of optical system in deep ultraviolet band are very complex. Therefore, we propose a method combining split design and single-layer diffractive optical element to realize the athermal design of deep ultraviolet optical system. In this method, the solution of the athermalized and achromatic equations are first presented, and the results are used to split and recombine the deep ultraviolet optical system, which simplifies the athermal design process. Then a single layer diffractive optical element is added to the combined system to simplify the design results. With this method, a deep ultraviolet reconnaissance camera lens with a focal length of 110 mm, an F-number of 3.5, and a temperature of -60--100 ℃ is designed athermally. When the Nyquist frequency of the obtained system is 18.5 lp/mm, the modulation transfer functions are all greater than 0.65. The results show that this method can solve the thermal defocusing of the deep ultraviolet transmission optical system in a wide temperature range, and at the same time can improve the optical performance of the system.