A pulsed laser based on master-oscillator power amplifier (MOPA) is reported.In order to get laser output of high repetition frequency and high average power，the Nd∶YAG thin slab was used，whose size is 4 mm×35 mm×160 mm and the Nd3+ doping concentration is 1%.Through side-pumped LD array for the immersion slab laser，stable laser output power of 4.15 kW is obtained when the average power of pump light is 15 kW，and the optical-optical conversion efficiency of resonant cavity is 27%.On the basis of that，two amplifiers were adopted to establish MOPA，the average output power of laser reaches 6.4kW.