The athermalization of a Schmidt optical system in the temperature range from -60℃ to 60℃ is designed by using wavefront coding. Taking a classical Schmidt system with a field angle of ±4° and a relative aperture of 1/2 as an object to be studied, a cubic surface is used as the wavefront coding surface. Under the constraint condition of a transfer function greater than zero, the parameters of the wavefront coding surface are optimized in a defocus range as large as possible. A Schmidt system with a defocus amount up to ±0.6 mm is obtained. Its focal depth is extended by a factor of 20 compared with that of the classical Schmidt system. Its defocused spot and transfer function data at different temperatures are analyzed. The result shows that this Schmidt system with wavefront coding has good adaptability in the temperature range from -60℃ to 60℃. Its optical performance is stable, so it facilitates the subsequent image restoration.