Based on master-oscillator power amplifier structure, an laser output with high pulse energy and high beam quality is achieved using a specific orientation Nd∶YAG laser amplifier. The laser amplifier includes three parts, a seed laser source, a pre-amplifier stage and a main-amplifier stage. In the main-amplifier stage, a laser diode side-pump Nd∶YAG rod amplifiers is used for the amplification of the seed laser. In order to obtain a high beam quality output, the thermal depolarization losses for different cut Nd∶YAG rods are simulated. According to the simulation results, the ［100］-cut Nd∶YAG rods are chosen as the active materials in the main-amplifier stage. Under the condition of repetition frequency of 200 Hz, pulse width of 25 ns, pulse energy of 40 μJ and a near diffraction limit seed laser injected, an output of 425 mJ pulse energy is gained with a beam quality factor of 1.37, and the output power stability of 0.81%.