Currently, spot welding technology is widely used in the overlap welding of galvanized steel plates for automobile applications. However, the novel laser screw welding technique is expected to gradually replace resistance spot welding because of its high efficiency and flexibility. In this study, the effects of inter-sheet gaps on the formation and mechanical properties of laser screw welding joints are investigated. The escape process of high-pressure zinc vapor and the causes of defects are analyzed at different gaps. For ST12 galvanized steel welded joint with 1.8 mm thick single-sheet and 6.7 mm spot weld diameter, this new technique offers an overlap gap window of 0.2--0.4 mm, suggesting wide adaptability for inter-sheet gaps. Using a gap greater than 0.1 mm, welding defects caused by the escape of zinc vapor can be effectively avoided. The best weld appearance and mechanical properties can be obtained at a gap of 0.2 mm because of the fluent escape of zinc vapor.