Abstract: Propellant tank pressures of rocket which is provided by the pressurization system are the key to decision success or failure of rocket. However the pressurization pressure are only defined according to a theoretical calculated method. In the past, the theoretical calculating method of the pressurization pressures was to adopt a traditional pressurization calculating equation. Now this article doubtes whether the formula is right, and proposes that it is necessary for us to discuss it again. On the basis of the low of conservation of energy, a new basic pressurization calculating formula has been derived, and then it has been proved by test that this new formula is conformable to practice while the error of the traditional formula is bigger.