Seam tracking is a significant precondition to obtain good welding quality. During the laser welding, the laser beam focus must be controlled to follow the welding seam accurately. A novel approach to detecting the offset between the laser beam focus and the welding seam based on infrared image processing is investigated during high-power fiber laser butt-joint welding of type 304 austenitic stainless steel plates at a continuous wave fiber laser power of 10 kW. The joint gap width is less than 0.1 mm. An infrared sensitive high speed camera arranged in off-axis direction of laser beam is used to capture the dynamic thermal images of a molten pool. The characteristics of thermal distribution and infrared radiation of the molten pool, when the laser beam focus is deviated from the welding seam center, are analyzed. Two parameters called the keyhole morphological parameter and the heat accumulation effect parameter are defined as the characteristic values of seam tracking offset to determine the offset between the laser beam focus and the desired welding seam. Also, the image processing technique is used to analyze the infrared images of the molten pool, which indicates the presence of mathematic correlation between the defined two parameters and the seam tracking offset. The welding experiments confirm that the offset between the laser beam focus and the welding seam can be estimated by the keyhole morphological parameter and the heat accumulation effect parameter effectively.