Abstract: In the basic Kepler telescope system, two pieces of coated glass are placed into the optical path at a 45angle with optical axis. One can reflect infrared light at the wavelength of 1550nm, while it transmits the visible and controlling infrared light with central wavelength at 1310nm; the other can reflect the controlling infrared light with central wavelength at 1310nm, while it transmits the visible light. By incorporating the controlling light and signal light paths, the system becomes simple and practical. Cost is reduced. The impact of the plate glass for visible light path is analyzed, and the path deviation falls within allowed error range by changing the thickness of the plate glass. Power loss of the signal light passing through some optical components is measured，the loss ratios for the telescope tube and plate glass measured at laboratory are 1.31% and 2.90% respectively. Transmission coefficient of signal light is deter-mined. Three optical path coaxial telescope system is successfully used in long-distance infrared control audio frequency monitor after many tests.