The research on laser lap welding of galvanized steels in automobile industry is very significant. The effects of gap on the features and mechanical properties of welding seam are analyzed. Meanwhile, the impact of setting small gap between faying surfaces on suppressing defects that caused by zinc vapor is studied and verified by experiments. The range of laser power and lap gap is also investigated. The outcome implies that for laser lap welding of HC260LAD+Z 100 MB galvanized steel sheet whose thickness is 1.35 mm, gaps exceeding 0.1 mm will suppress the defects caused by zinc vapor effectively. Penetration reaches the maximum value when the gap is about 0.15 mm. Greater laser power of 4 kW, has better tolerance on gap from 0.05~0.2 mm.