Aiming at the calculation problem of vehicle exhaust emission in the pollution gas monitoring system, a gas emission calculation model based on the optical flow method is proposed in this paper. The two infrared cameras are applied to capture the multi-frame gas image sequences. Optical flow method then is used to obtain the optical flow velocity field matrix of the gas plume. Finally, we obtains the instantaneous gas displacement by combining the optical flow velocity field matrix with a two-dimensional gas concentration map. By using different optical flow methods for experimental comparison, Farneback optical flow method has better adaptability in the model and can achieve a higher processing effect. The maximum fitting degree of gas displacement calculated by the optical flow method reaches 93%. The operation speed is also better than Lucas-Kanade optical flow method and Horn-Schunck optical flow method, while the accuracy is maintained. In calculating low emission speed and high emission speed gas, Farneback optical flow method achieves more reliable stability. A new idea for non-contact gas monitoring is provided.