In calculating infrared radiation transmission,it often has large deviations by only using empirical formula.It is complicated to use MODTRAN and difficult to transplant it to other infrared systems.Therefore,by using molecular single absorption method a new calculating method is proposed based on lots of measured atmospheric data,absorption coefficients of water vapor at different temperatures.The analysis formula of temperature vertical distribution was fitted in different months.Spectrum transmittance and average transmittance were simulated and calculated.The results show that height、zenith angle and distance have great influence on the transmittance of water vapor.The transmittance calculated by the method is accurate and has referent value in the project application.