As the operating conditions of motor vehicles are complex and changeable, the concentration range of exhaust components is very large. Due to the fixed structure of optical gas cell and the detection limit of the system for weak photoelectric signal, the concentration range and detection accuracy of the gases to be measured are greatly limited when using conventional optical methods to detect the concentration of pollutants in exhaust gas. It is found that based on Lambert Beer′s law, adding an exponential factor correction to the gas concentration variable to be measured can realize the large range detection of CO and CO2 without reducing the measurement accuracy. The calibration experiment of the portable vehicle exhaust detecter with standard gas shows that the fitting degrees of CO and CO2 obtained by the traditional linear correction method are 0.988 and 0.998 respectively, while those of CO and CO2 obtained after adding nonlinear correction factor are 0.999 and 0.999 respectively. Further field comparison shows that the measurement results of the modified instrument are in good agreement with those of similar advanced instruments. The experimental correlation of diesel vehicle is 0.93 and that of gasoline vehicle is 0.95, which verifies the necessity and practicability of the proposed nonlinear correction method.