To address the issue of limited detection of water vapor in the near-surface area by existing Lidar technology, a side-scanning continuous light method is proposed. This method uses a high-power continuous light laser as the detection source and obtains the water vapor concentration contour by side-scanning the light scattering and absorption signals on the detection path. The water vapor concentration contour is then obtained through inversion based on the differential absorption method. The model establishes the detection of water vapor concentration using side-scanning continuous light. The validity of the method is verified through simulation, which shows that semiconductor continuous-wave lasers with detection wavelength pairs of 880 nm and 940 nm and energy of 5 W can effectively detect water vapor within 500 m near the ground. This method does not require expensive narrow-pulse lidar as the detection light source. It has the advantages of being easy to implement and cost-effective. It can be a useful supplement to existing water vapor detection technology for near-surface detection.