Abstract: An approach of detecting the offset between the micro-gap seam and the laser beam focus based on wavelet transform was presented during fiber laser butt-joint welding of Type 304 austenitic stainless steel plate with a high power 10 kW continuous wave fiber laser. This offset is less than 0.1 mm. To study the details of seam offset characteristics, an infrared sensitive high-speed video camera was used to capture the dynamic images of the molten pools. Multi-scale decomposition was implemented to the interested area of a molten pool image by using the wavelet function. The collected horizontal coordinate of high frequency coefficient was defined as a keyhole deformation parameter, and then this keyhole deformation parameter was deemed as a keyhole eigenvalue. Through the analysis of the relationship between the eigenvalue and welding seam offset, it was found that the offset between the micro-gap seam and the laser beam focus could be estimated by the keyhole deformation parameter effectively.