Through the theoretical analysis on critical ratio of the throat to port area J in a solid rocket motor, the authors believe that there is a maximum J,,,. For different ratio of specific heats k, the value of J,,, is about 0.56. If the designed J is greater than J,,, the chamber pressure will be P-omin greater than the designed pressure pd. The theory is further confirmed by the comparing of calculation with a serial experiments in a real rocket motor.