Green laser with large pulse width and high single pulse energy was developed, which has a pulse repetition frequency synchronized with high speed camera frame frequency and is applicable for fiber coupling output. Together with the application of plano-convex unstable resonator, the laser was equipped with a flash-lamp pumped Nd∶YAG crystal, a KTP crystal for intracavity frequency doubling and a passive Q-switching crystal. A laser light output was realized with a wavelength of 532 nm, the maximum repetition rate was 300 Hz, the pulse width was 70 μs, the average power was 38 W, the single pulse energy was 126.7 mJ and the divergence angle was 3.5 mrad. The laser was further coupled to an underwater fiber of 800 μm in diameter, achieving a 92% coupling efficiency.