In order to measure the velocity of projectiles in larger testing section and higher precision,a diagnose system for projectiles velocity based on light meshs has been developed.A LD laser,the wavelength of which is 980nm,has been used as a light source in the system.Light shunttors and collimators have been used to form three light meshs.Each mesh has seven light beams.The velocity of projectile can be calculated by using times of light beam shunt off by projectile and intervals of light meshs.The testing section of system is 100mm×100mm,testing range is 50~4000m/s,measuring error is less than 0.1%.This system has been successfully used to measure the projectile velocity of an electromagnetic rail gun.