The MOPA nanosecond pulse laser was used to weld 316 stainless steel, and the influence of pulse width, frequency and welding speed on weld formation was explored, as well as the mechanism. It was found that a smooth weld seam without porosity defectcould be easily acquired at high laser pulse frequency, which played a key role in welding thin sheets. At the range of 30-120 ns pulse, the weld depth increased with pulse frequency to the maximum at central pulse frequency, and then decreased. While for the longer 200-500 ns pulse, the weld depth decreased as the pulse frequency increasing ( >100 kHz). According to the cross-section morphology at the different pulse frequency, there were two different welding regimes: the keyhole (or deep penetration) and conduction regime. It was suitable to welding stainless steel sheets at long pulse and high frequency controlled by conduction mode, resulting in sound weld quality.