A compact diode-pumped double-pass N d:YVO_4slab laser amplifier is realized.The master oscillator is passively Q switched microchip laser,and 0.5W of averag e power at 20kHz,2.4ns of pulse duration is output in TEM₀₀.In order to achieve high efficiency and high gain amplification outp ut,we designed a compact structure to collimate the pump laser and the seeder laser.In order to optimize the energy e xtraction and to reduce the thermal aberrations,the pump laser is collimated by a cylindrical microlens in the ver tical direction.In order to match the horizontal gain diameter,the cylindrical lens (f=60mm) is used to focus the laser into the slab in the first pass.A spherical lens (f=75mm) is used as the phase tr ansfer system to match the mode volume of the pump laser.After double-pass amplification,average output power of 6.3W at 20kHz, pulse duration of 2.5ns and M^2of 1.5is achieved,corresponding to 16% extraction efficiency.