Abstract: his paper has introduced the basic principle, structure and operation instruction of the measuring equipment for studying cryodeposite on optical surfaces.The main performances have been obtained with experiment. System errors have been analysed. This device is simple in structure, complete in function, reliable enough in performance and convenient in use. The measuring wavelength is from near infrared to far infrared and the precision is better than 3%, which is close to the same equipment from external. It can be used for measuring all kinds of condensable vapour cryodeposite. It will be an effective device for further developing space technology.