Cryogenic deuterium pellet injection is one of important techniques for edge localized mode (ELM) mitigation for successful operation of ITER in the future. A high frequency, 50 Hz, pellet injection system (PI-50) based on screw extrusion forming ice and pneumatic acceleration has been built and developed for ELM mitigation for EAST. As velocity and reliability of launching frozen pellet are of importance to ELM mitigation, a diagnostic system including video cameras is installed on this pellet injection system to acquire the pellet's velocity and monitor the quality of pellet. With the help of this diagnostic system, the velocity and reliability of PI-50 system have been measured successfully. The reliability of PI-50 is more than 85% when the launching frequency is 50 Hz, which means this system can be used for ELM mitigation on EAST in the future. PI-50 system has been installed on EAST at present and is expected to be involved physics experiment in next campaign on EAST.