Laser amplifiers with high pulse energy and narrow pulse width can be used in many fields, such as material processing, medical plastic surgery and lidar. The Master Oscillator Power Amplification (MOPA) technology, which combining the seed laser and the amplifying structure, not only enables features of the output pulse laser are consistent with that of the seed laser (pulse width and repetition frequency, etc.), but also ensures the amplification of the output laser energy. Thus, MOPA technology becomes the main technology in laser amplifier based on engineering applications. In this subject, a laser amplifier based on a sub-nanosecond microchip solid-state laser was developed in response to the demand for sub-nanosecond high-energy laser amplifiers in medical cosmetology. Firstly, a sub-nanosecond passive Q-switched microchip solid-state laser was used as the seed source. The seed source laser has a repetition frequency of 10 Hz, a pulse width of 487.3 ps, and a wavelength of 1064 nm to output seed light with an energy of 190 μJ. Then two self-made xenon lamp-pumped Nd:YAG modules were used as the main amplifier to amplify the sub-nanosecond laser pulse energy. The energy generated from the self-oscillation in the amplification process was suppressed, and the energy transfer efficiency during the amplification was increased effectively. Finally, a switchable output with wavelengths of 1064 nm and 532 nm was obtained. When the repetition frequency was 10 Hz, a sub-nanosecond laser output with a pulse width of 496.4 ps, a pulse energy of 561 mJ@1064 nm, 330 mJ@532 nm, an energy stability of 2% and a uniform spot were obtained.