Abstract: The water vapor transmittances in the 940 nm band are computed using MODTRAN for 34 sites in China.Parameters a and b for all the atmospheric models in the empirical water vapor transmittance function are fitted.It is found that the values of a and b is the function of atmospheric water vapor amount.The empirical functions are established for the relationships between the values of a and b and atmospheric model's perciptible water.Considering the parameter of the surface altitude H(or surface pressure p0),the empirical coefficients related to H are parameterized.Finally,the uncertainty in retrieved PWV caused by parameter a and b is analyzed.