An image analysis based TO focal length test and automatic laser welding method is proposed through BOSA device structure analysis. TO moves every single step up from an initial point driven by a stepper motor. An image acquisition system is utilized to collect spots images of TO focal point. The optimal position of TO focal point, which is located on the bonding surface of mortise and round square tube, is obtained through OpenCV based image analysis methodology. TO is then fixed by laser welding. Experiment results show that the system has high accuracy and good practicality.