Oscillating laser beam was utilized to weld 5 mm thick 1Cr18Ni9Ti stainless steel, and the joint with good appearance and low porosity was obtained. The effects of welding speed, oscillation amplitude, oscillation frequency, and oscillation pattern on weld formation and porosity were analyzed. The main mechanism of porosity suppressing was revealed from the perspectives of keyhole, molten pool flow and bubble overflow. The research results show that appropriately higher value of oscillating amplitude and welding speed were more effective for porosity suppression. There is an optimal range of oscillating frequency to get low porosity. For 5 mm thick stainless steel, when the laser oscillation frequency is 100 − 300 Hz, lower porosity and better weld formation could be obtained. Better weld formation were obtained with the lowest weld porosity of 2.94%, using the “8” shaped oscillating laser. While the worst weld formation was obtained with higher porosity of 19.13% using linear oscillating laser.