The defocus distance is one of the most important factors during filling laser welding with wire. Keeping defocus distances stability is the pre-condition to acquire the stable weld quality. In this paper, a QUANTA laser camera was used to measure the defocus distance, and it is compensated in real time through the numerical control system. The results reveal that the fixed defocus distances can be acquired using this system. And the experiments by using auto-controlling method of the defocus distance show that the stability of welding process can be improved greatly and the good weld quality can be easy to be obtained.