The relation of grain size with progressive combustion of the propellant was studied by closed bomb test. The characteristics of p-t and L-B curves for different prescription and grain size of the variable burning rate propellant were analyzed, and the influence law of grain size on burning properties were obtained. Results show that, for certain prescription and thickness for the inside and outside layers of the variable burning rate propellant, the ratio of length to diameter (1.5/1-2.0/1) affects the progressive combustion more obviously. The ratio of length is higher, the progressive combustion is better.