Because of some shortcomings for the current atmospheric transmissivity calculation methods used in the real-time simulation of infrared imaging, a new algorithm for calculating the atmospheric transmissivity in the real-time simulation of infrared imaging is proposed according to the real-time and accurate requirements of transmissivity calculation. The new algorithm is simulated by using the Visual C++6.0 programming language. The infrared simulation and experiment evaluation under different conditions have shown that the algorithm has a good feasibility.