The austenitic stainless steel is welded with the fiber laser and coaxial protection method. The optical signals during the welding process are collected by the laser welding and synchronously monitoring system, which is combined with the shape of plume to study the influence laws of different welding conditions on the weld formation and optical signal intensity. The results show that the optical signal intensity gradually increases with the increase of laser power. When the defocusing amount changes from -6 mm to 6 mm, the optical signal intensity decreases first and then increases. The optical signal change can reflect the change of weld penetration, which can be used to detect the weld defects caused by the change of the lap gap and the weld position. The exact location of the area with the abnormal welding quality can be judged by analyzing P signal data. The optical signal intensity is positively related to the plume volume during the welding process.