The differential optical density can be obtained through several high-pass filtering methods based on the differential optical absorption spectroscopy(DOAS). Concentrations of different gases can be calculated by least square fitting method using the standard reference spectrum which is processed with the same digital filtering. Several kinds of smoothing filtering and polynomial filtering methods are used to deal with the same spectrum signal. Accuracy of the gas concentration, the standard deviation of the residual, and peak-peak values are compared. The results show that the high pass filtering method using 3-500 smoothing filtering which is repeated twice is much better than other methods. It can effectively improve the measurement accuracy and reduce the lower limit.