To solve the problems of multiple water gauge stitching, incomplete water gauge shooting and poor environmental adaptability in water level recognition of water gauge images, a photogrammetric method is proposed to measure the water level of indefinite water gauge image under complex lighting conditions. Firstly, the perspective projection relationship of the water gauge between the pixel coordinate system and the world coordinate system is constructed through the manually selected identification points. Then, the water gauge and the background are segmented and binarized by the color characteristics of the water gauge. According to the binarized image, the average and variance threshold method is used to detect the position of the water level. Finally, the expression of the middle line on the water gauge in the water level image is calculated through the projection relationship. The length of the middle line on the water gauge on the water surface is calculated by combining the position of the water level line to obtain the water level value. In this paper, the measurement experiment is carried out on the real water gauge image of the hydrological station, and the detection results are compared with the remotely measured water level value. Results show that this method could improve the reliability of the water level detection of the indefinite water gauge image. The measurement accuracy is up to 1 cm and the detection resolution is 1 pixel, which basically meet the water level monitoring requirements of hydrological stations.