An experimental study on optical phase conjugation is developed in a double-pass high repetition rate laser diode pumped Nd:YAG amplifier. The traditional 0° HR mirror is replaced with a quartz glass rod phase conjugator. A stable phase conjugating reflection is realized at 400 Hz repetition rate with a 20 ns pulse width input. As a result, the beam distortion caused by the thermal effect of the gain medium is well compensated, and a better amplified output beam profile with no diffraction ring is observed. The output beam M2 is improved from 4 to 1.4. The optical breakdown or injury in quartz glass is avoided.