Abstract: Temperature structure coefficient (C) and velocity structure coefficient (C) can be deduced from the sodar return intensity. The past researches are reviewed briefly. The impacts of these physical factors, such as excess attenuation, wind and turbulence etc on the echo measurements are discussed. The results show that the excess attenuation of acoustic wave is mainly caused by the velocity fluctuation. On the other hand, the wind also causes some echo attenuation. When wind speed reaches 10 m/s, the received energy at antenna will reduce about half. At the end, a modified sodar equation and the retrieval method of C and C are given. The primary tests indicate a qualitative agreement with the theory presented here.