To overcome the shortcomings of high cost, low precision and inconvenient placement of traditional gas sensors, a set of opposite laser humidity sensing system is constructed. The system uses a semiconductor laser as a light source. A light transmitting unit and a light receiving unit are respectively arranged on both sides of a measuring cabinet. When a laser beam passes through the measuring portion in the middle of the measuring cabinet, the humidity value of the portion can be measured. The experimental results show that the calibration coefficient R2 of the opposite sensing system is 0.9976. The retesting errors of the humidity content and the theoretical calculation value in the three-meter optical path are 0.82%, 0.46%, 0.44%, and 0.42% respectively. Within the 40-minute test after the instrument stabilizes, the humidity content range in the actual test cabinet is fluctuated to a maximum of 30. The standard deviation is 7.88 and the relative error is 0.4%.