The present invention relates to a device and method for measuring the amount of a liquid chemical in a plant protection unmanned aerial vehicle (UAV). An ultrasonic sensor is adopted to measure a distance between a liquid chemical level in a liquid chemical measuring tube communicated with a liquid chemical tank and a top end of the liquid chemical measuring tube, and then a height of the liquid chemical in the liquid chemical tank is further determined according to an acquired attitude angle of the UAV and a distance between a central position of the liquid chemical measuring tube and a central position of the liquid chemical tank, thereby accurately measuring the amount of the liquid chemical in the liquid chemical tank of the UAV.