An automatic calibrated deep foundation pit water level measuring instrument was designed， which was composed of a controller， a laser sensor， a controllable lifting motor， an alloy design board， etc. The controller controlled the lifting of the master-slave motor to form an angle difference. The laser sensor was in a certain dimension. The inner surface of the deep foundation pit was scanned to obtain the measurement data， and then the relevant model was established to calculate the angle between the sensor and the water level. The controller controlled the laser sensor to be perpendicular to the water level to realize the self-calibration function. This design had overcome the drift of measuring points caused by some external factors such as vibration and body offset， so as to achieve the purpose of accurate measurement.