Abstract: The test results show that artillery rockets will achieve better accuracy when the high-low pressure recoilless launchers are used to fire in the launch tube. This paper established the internal ballistics equation for the high-low pressure launcher with tandem construction when the rocket motor is ignited in the launch tube. Numerical computations have been done for the under-test projectiles. The paper also discussed the influences of a number of factors on the internal ballistics，such as the initial volume of the low pressure chamber，the position of the rocket ignition，the relation of the front jet area and the tail nozzle throat area of the high pressure chamber，etc. The results of the computation are in good agreement with the results of the test.