The embodiment of the invention discloses a calibration coefficient determination method and calibration method and device for a flowmeter on an unmanned aerial vehicle. The calibration coefficient determination method comprises the steps that when it is determined that all to-be-sprayed substances carried on the unmanned aerial vehicle are completely sprayed, a flight controller acquires the cumulative measurement flow rate which is fed back by the flowmeter and corresponds to to-be-sprayed substances; according to the actually carried amount of the to-be-sprayed substances and the cumulativemeasurement flow rate, the flight controller determines calibration coefficients of the flowmeter. The flight controller can calibrate the cumulative measurement flow rate fed back by the flowmeter in real time according to the calibration coefficients of the flowmeter during official plant protection operation of the unmanned aerial vehicle to obtain the actual spray flow rate after calibration,and in this way, calibration of the flowmeter on the unmanned aerial vehicle is achieved. According to the technical scheme, the problem that the flowmeter is difficult to calibrate during the plantprotection operation of the unmanned aerial vehicle is solved, the calibration process is convenient and simple, the accuracy of spray flow data obtained by the flight controller is ensured, and the effect of the plant protection operation of the unmanned aerial vehicle is also ensured.