The invention relates to a device and a method for filling and unloading liquid ammonia. The device comprises a liquid ammonia buffer storage tank, a liquid ammonia filling and unloading arm and a delivery pump, and is characterized in that the liquid ammonia buffer storage tank is connected with the delivery pump; the delivery pump is connected with a liquid phase main pipeline; an outlet of the delivery pump is further provided with a backflow pipeline; a backflow regulating valve is arranged on the backflow pipeline; the liquid phase main pipeline passes through a rapid cut-off valve, a flame arrester, a mass flowmeter, a regulating valve of the mass flowmeter, a manual ball valve and the liquid ammonia filling and unloading arm on a liquid ammonia pipeline in sequence; the mass flowmeter and the regulating valve of the mass flowmeter are connected with a filling and unloading quantitative control system; a tank car is further connected with a gas phase filling and unloading arm; the gas phase filling and unloading arm is connected with a gas phase main pipeline or a compressor via a pipeline provided with a pressure meter, a flame arrester and a pneumatic regulating valve in sequence; the gas phase filling and unloading arm is then connected with an ammonia buffer storage tank; and the liquid phase main pipeline is further connected with the gas phase main pipeline via a safety valve. The device and the method can be applied to the field of filling and unloading various easily liquified liquid such as liquid ammonia, dimethyl ether and natural gas.