The key of forest fire prevention is to predict the fire risk effectively through the real-time fire monitoring system. By analyzing the application of UAVs in forest fire prevention, a medium wave infrared camera system for forest fire real-time monitoring based on UAVs is proposed in this paper. The focal length is 75 mm, the wavelength band is 37--48 m, F number is 20. The system can match a 1280×1024 cooled MWIR focal plane detector with pixel size of 12 m×12 m. In the optical design, three infrared optical materials (silicon, zinc selenide and germanium) are combined, and three even aspheric surfaces are introduced. The passive athermal optical design is realized as well. The design results show that the modulation transfer function (MTF) of each field of view is greater than or close to 04 at the spatial frequency of 417 lp /mm in the temperature range of -60~100℃, which has good imaging quality. Since the medium wave infrared system has high imaging quality and resolution, large field of view, adaptability in wide temperature range, compact structure and can be installed easily，it has a wide application prospect in the field of forest fire prevention.