In this paper the construction and operation characteristics of a high power Nd r YAG laser system are described. The laser system consists of an active-passive mode-looked oscillator in colliding pulse regime, a single pulse selector and a fourpass amplifier. The oscillator generates mode-locked pulse trains with widely variable pulsewidths (20ps～2.5ns) and a fluctuation of the pulse-train envelope <±4%. For a 0.1mJ, 200-ps laser pulse, the four- pass amplification of 7×102 has been achieved.