The liquid-level data is one of the most important informations for flying operators of spaying UAV. During the operation, the liquid content in the pesticide tank is dynamic, the flying operators have always to pay attention to the liquid content so that they can make suitable decisions for the flight control according to the liquid content adjustment in the pesticide tank. Therefore, the liquid-level monitoring method for pesticide tank is important in the design of key components of spraying UAV. In order to explore the feasible method for liquid-level monitoring of pesticide tank in spraying UAV, several major methods about contact and non-contact liquid-level monitorings were reviewed, including differential pressure, floating electrode, capacitance, ultrasonic, laser, photoelectric, flowmeter, machine vision and radar. Spraying UAV has special requirements, such as liquid surface violent fluctuation, different physical and chemical properties of different pesticides, different shapes in different tanks and high anti-corrosion requirements and small size of tank. Lots of limitations of the above mentioned methods when they were used in spraying UAV were pointed out. And then solutions called air-pressure wireless liquid-level monitoring (APWLLM), a kind of non-contact liquid-level monitoring method, which was suitable for use in spraying UAV, were proposed. It would provide a reference for the further development of liquid-level monitoring device for pesticide tank in spraying UAV.