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The porosity in the welded seam can be generated easily during the YAG laser nonpenetration welding of zircaloy sheet, which affects the quality of welding. This research used Zr-4 and N18 as the objects for the welding experiment. The method of cutting cross-section of weld seam was used to analyze the porosity number and observe the morphology and location of porosity in the weld. The effects of process parameters including laser pulse current, pulse width, defocusing distance and laser pulse modulation on porosity generating were discussed. The results show that the generating of porosity is due to the unstable collapse of the keyhole in the process of YAG laser nonpenetration welding of zircaloy. The porosity would be formed when the speed of bubble escaping from the weld pool is lower than that of melting metal solidifying. Under the condition of 1.0 mm penetration welding, the results also show that with the increase of laser pulse current and pulse width, the porosity number presents a gradual increase. With the increase of defocusing distance, the porosity number shows a gradual decrease. Compared with non-segmented programming model, this research uses such measures as segmented programming, slowing down the current and reducing welding speed to obtain the relatively lower porosity number, and the pore size can be controlled below 0.5 mm effectively