A experimental study about a triple-pass grazing incidence Nd∶YVO4 slab laser amplifier is carried out based on a semiconductor saturable absorber and passively Q-switched microchip lasers. Influences of the mode matching ratio in horizontal direction and grazing incidence angle on thermal distortion of slab laser amplifier are analyzed theoretically. The experiment verifies that the mode matching ratio of the grazing incident slab laser amplifier in horizontal direction is optimized to be 0.6. Meanwhile, with a small grazing incidence angle, thermal distortion in horizontal direction can be alleviated and high gain can be obtained. When the repetition rate is 100 kHz and the seed source is with an average power of 2 mW and a seed beam quality factor M2 of 1.17, a laser output of 12.6 W can be obtained after triple-pass amplification of grazing incidence Nd∶YVO4 slab with a mode matching ratio of 0.6 and a triple-pass grazing incident angles of 3°, 5° and 7°, respectively. The optical-optical extraction efficiency reaches 28%. The peak pulse power is 1.4 MW, the pulse energy is 126 μJ, and M2 is 1.4.