In projectile's launching process in the bore, it will bear bad environment of high pressure, high temperature, high overload. In order to accurately measure the pressure of projectile from the gunpowder gas at the bottom of the bore, a small volume of built-in pressure measuring instrument was designed, its measuring principle and realization method were also expounded. The measuring instrument uses storage measuring technology, high strength vacuum potting technology and high efficiency buffer technique, and through the calibration and assessment test under the simulation application environment, successfully acquires pressure data at the bottom of the projectile and chamber of projectile in the process of launching. The study plays an important role in the design of fuze, projectile and checking acceptance of gun.