A double-pass configuration of MOPA system is realized by a diode pumped ,single longitudinal mode Nd∶YAG master oscillator and three flash-lamp-pumped YAG amplifiers with a phase conjugating stimulated-brillouinscattering cell. After single - pass amplifiering of two amplifier cells ,the laser output energy of 6. 58J , pulse duration of 3. 6ns ,divergency angle of 1. 7mrad and 10Hz repetition rate ,the peak power is 1. 82GW.