The invention provides a propellant surplus measuring system. The propellant surplus measuring system comprises a gas cylinder, a pressure sensor, a self-locking valve, a differential pressure gauge and a propellant storage tank. The invention further provides a propellant surplus measuring method. The method comprises the steps of setting the state of the self-locking valve; controlling the self-locking cylinder to enable the initial states of the two gas cylinders and two storage tanks to be consistent; controlling the self-locking valve to discard one gas cylinder, keep the state of another gas cylinder, pressurizing one storage tank and keeping the state of another storage tank, and recording the pressure of the measuring system and the difference value of the temperature gauge; controlling the self-locking valve, and reducing the initial state of the system; and calculating the surplus volume of the propellant according to the recorded pressure, temperature and pressure difference value of the measuring system. By adopting the method provided by the invention, the air-capacitor configuration of a primary gas injection method can be saved, and the measuring times can be improved greatly and can reach up to more than 50 times.