According to the charecteristics of the backscattered light from the rough surfaces of the target far away and the random phase matching rule, the method of coherent optical head coefficients designing is discussed. An optical system is developed that is an optical head of a coherent imaging CO2 laser radar with chirp pulse compression. In the experimental laser radar only one continuous wave single mode CO2 laser is used, and the signals from a building of 7.8 km away are obtained with acceptable SNR while the lase output is about 5 W.