The　experiment　of　sub-nanosecond　pulse　width　Nd:YAG　double-pass　amplification　is　studied.　Double　pass　amplifier　gain　medium　is　Nd:YAG　rod　with　φ3　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.