An injection-seeded single-frequency Q-switched Nd:YAG laser is accomplished by using a phase modulated rampfire technique.A RbTiOPO4 (RTP) electro-optic crystal is selected for effective optical path length modulation of the slave self-filtering unstable resonator.This single-frequency laser is capable of producing 50 mJ pulse energy at 1 Hz repetition rate with a pulse width of 16 ns.The standard deviation of laser pulse intensity for consecutive 100 shots from the mean pulse intensity is less than 1.05％.A spectral linewidth of less than 0.5 pm with a frequency jitter of about 14 fm over 30 min is obtained.