The experiment of sub-nanosecond pulse width Nd:YAG double-pass amplification is studied. Double pass amplifier gain medium is Nd:YAG rod with 3 mm×120 mm size and doping atomic fraction of 1.0%. The two end surfaces of Nd:YAG rod with an angle of 1.5° is designed to avoid self-oscillation and amplified spontaneous emission (ASE) effect. The beam expanding lens magnification is optimized appropriately, the seed laser spot area is selected about 80% of Nd:YAG rod surface area to obtain a higher efficiency. The master oscillator output laser with single pulse energy of 0.16 mJ, repetition frequency of 5 Hz, M2 of 1.5, and pulse width of 0.964 ns is obtained. The Nd:YAG double-pass amplifier output laser with single pulse energy of 88 mJ, M2 of 1.7, stability of ±3%, pulse width of 0.975 ns has been obtained and the total gain is 550.