Abstract: In order to obtain physical mechanism of plasma generated by hypervelocity impact, the energy distribution of hypervelocity impact of an LY12 aluminum projectile into an LY12 aluminum target was analyzed in all the physical processes, related to energy consumption of melting phase transition, vapor phase transition and plasma formation. Factors influencing vaporization and plasma formation were disclosed in every physical stages during the formation of impact ejecta. In addition, additional heating mechanism and physical mechanism of formation and expansion of plasma plume were included after impacting. The model description of every physical process was also given.