In order to study the burning rules of gun propellant in gun chamber,feasibility of microwave interferometer testing and ordinary testing was analyzed based on the classical interior ballistic theory,and a relative calculation method was proposed. p-t curves and muzzle velocity were considered as basic data by the ordinary testing method,however v-t curves was considered as basic data by the microwave interferometer testing method. The burning rate of gun propellant in gun chamber could be obtained by both methods. Results show that the calculated p-t curves are consistent with the p-t curves of testing. Under pressure between 80 MPa and 250 MPa,the range of burning rate is between 17 cm?s^-1 and 29 cm?s^-1 by ordinary testing method,and under pressure between 80 MPa and 250 MPa,the range between 8 cm?s^-1 and 23 cm?s^-1 by microwave interferometer testing method. The formula of gun propellant burning rate in gun chamber deviates far from the exponential function. The precision of microwave interferometer testing method is better than that of ordinary testing method.