To solve the problem of weld defects caused by zinc vapor in the process of laser welding of galvanized steel sheets, we adopt the laminar flow above a keyhole to keep the keyhole open continuously and form a stable zinc vapor venting channel. The effects of the laminar gas flow on weld forming quality and mechanical properties of gap-free laser lap welded joint of galvanized steel sheet are studied. The influencing mechanism of the laminar flow on the effective escape of zinc vapor is analyzed with the utilization of real-time monitoring and numerical simulation of the welding process. The results show that for gap-free laser lap welding of DP590 galvanized steel sheet with thickness of 1 mm, the laminar flow can effectively promote the escape of the zinc vapor, and thus a lap joint with excellent surface quality and mechanical properties is obtained.