A Fourier transform infrared (FTIR) spectroscopy system based on the sun tracking method is developed to measure the composition of atmospheric pollution gases. The system consists of a solar tracker, an optical transmission section, and a spectrometer. An orthogonal mirror system, which uses the principle of pinhole imaging to realize sun tracking, is designed for collecting sunlight, ensuring that the tracking optical path is coaxial with the measuring optical path. The theoretical calculation formula of the spot trajectory on the position sensitive detector (PSD) during tracking rotation is derived to guide the PSD algorithm. The working band of the system ranges from 600 cm－1 to 5000 cm－1, and the resolution is 0.5 cm－1. The influence of the focal length of the parabolic mirror used to converge the interference beam on the interference fringe is analyzed using the optical software Zemax. The focal length value is determined to be 52.5 mm, the maximum tilt angle of the incident light under the requirements of the system index is determined to be 0.118°, and the technical indexes of measurement accuracy of the PSD and tracking accuracy of the system are also given. Furthermore, the preliminary outdoor experiment is carried out by using the developed experimental platform, and the rationality of the system is verified.