In order to study effect of defocus on droplet transfer and related characteristics of laser welding with filler wire, and achieve stable droplet transfer mode, a high speed camera system was used to observe droplet transfer under different defocus parameters. Droplet transfer was divided into three kinds: liquid bridge transfer, mixed transfer and drop transfer. The results show that, liquid bridge transfer mode with defocus of -1mm and +3mm could improve the stability of welding process with good quality, such as the weld cross section without porosity. For the drop transfer mode with defocus of +5mm, the welding stability is the worst. During the welding process, keyhole is completely closed, the surface of weld bead is formed irregularly, and the large pores appear at the bottom of the weld corrosion-face. The research results have guiding effect on actual production.