The invention discloses an automatic filling device and method of a fluid loop. The device and the method are characterized in that a pressure reduction valve and a manual ball valve on a pipeline ofa raw gas channel of the filling device are correspondingly replaced by an electric pressure reduction valve and a solenoid valve; a manual adjusting valve on an original fluid returning pipeline of the filling device is replaced by an electric adjusting valve; a flow rate switch is mounted on a pipeline through which a water gauge is connected to an overflowing groove; and other manual ball valves on the pipeline are replaced by electric ball valves. With the adoption of the device and the method, automation can be realized; human resources can be reduced; and the filling accuracy can be improved.