The forward small-angle solar transmittance ratio is more sensitive to ice crystal particles in cirrus clouds compared with aerosol particles. A photometer, named VFOVSP, is developed based on image tracking and automatic fast changing field of view, which can quickly measure transmitted radiation from narrow field of view to wide field of view of the sun. It provides a new technology for ground-based measurement of cirrus. The system composition and measurement principle of the instrument are introduced. In the atmosphere non-absorption band,after the instrument is calibrated with Langley method,the VFOVSP measurement results are compared with the measurement results of POMO2 photometer to verify the reliability of the instrument measurement accuracy. Under different weather conditions, the experimental results show that the transmission ratio at different small angle fields of view is related to the type of particles, which makes it possible to distinguish thin cirrus clouds and aerosol particles. The instrument makes up for the lack of real-time and single-field detection of traditional solar photometers in cloudy weather. It can better identify whether there is cirrus under the current atmosphere and meet actual scientific research needs.