A single longitudinal mode(SLM) pulse from a Q-switched laser, after being attenuated and slied, was injected into a Nd:YLF Q- switched slave Cavity. Its energy was amplified to 1. 5 mJ from 1. 2 nJ and its pulse width was compressed to 4. 5 us from 6 ns after amplification. A gain of 106 of the amplifier is reached. It was tested and verified by our experiments and theoretic analyses that the match of cavity lengths between a master cavity and a slave cavity was unnecessary if the pulse width of an injecting seed is shorter than a round trip time of the slave cavity. The course should be called regenerative amplification of SLM laser, not injected-locking.