In order to detect formaldehyde gas in severe chemical pollution area, the research of concentration detection for formaldehyde by differential absorption lidar (DIAL) is carried out in this paper. Based on the principle of differential absorption lidar and the strong absorption of formaldehyde gas in the mid infrared band, the detection wavelengths of ${\lambda _{\rm on}}$ and ${\lambda _{\rm off}}$for the formaldehyde are selected in consideration of the influence of atmospheric interference gas under open optical path. According to the concentration inversion method of differential absorption lidar and combined with the system parameters, the effects of the detection distance, gas concentration and interfering gas on the system perform are studied. The results show that the system can detect formaldehyde concentration in the range of 0.017-1.5 ppm (1 ppm=10^-6) and within the distance of 0.4-1.1 km, and the relative error is less than 5%, which can meet its demand for formaldehyde detection in severe chemical pollution area. This paper can provide a theoretical and technical basis for the development of mid infrared differential absorption lidar system for monitoring the unorganized emission of formaldehyde in severe chemical pollution area.