In order to improve detection range and environmental adaptability of a toxicant light detection and ranging (LiDAR), a long-wave receiving optical system with refractive-diffractive structure was designed by using passive athermalization. By utilizing unique chromatic aberration characteristics of a binary diffraction surface, the chromatic aberration and secondary spectrum of optical system were corrected. Under the condition that the working wavelength was from 9 μm to 11 μm, focal length was 270 mm, F number was 2.27, and optical efficiency was higher than 86.6%, tolerance analysis was conducted with Monte Carlo statistical method. By conducting temperature and flight tests to measure the key technical indicators, the simulation results and design rationality were verified. The results show that the wavefront root mean square of optical system is 0.03λ, and the modulation transfer function is higher than 0.47 in the temperature range of -40 ℃~60 ℃, so the image quality is excellent. In flight test, the maximum detection distance of toxicant LiDAR that applies this optical system reaches 4.05 km. The result is superior to system requirements, so the designed optical system is qualified. The study provides positive effects on development of laser active telemetry radar for biochemical environment.