The medium-Mn steel (0.1%C-5%Mn-Fe), renowned for its exceptional balance of high strength and ductility, has garnered significant attention in the automotive industry as a promising material for next-generation automotive steel. The medium-Mn steel was joined with fiber laser, and formability of the welded joint was carried out with the Erickson cupping test machine. The factors affecting the cupping test performance of the joint were analyzed with finite element simulation, and the cupping test results were compared with those of gas tungsten arc welding. The results show that "hardening zone" is formed in welded joint, which reduces the formability of the welded joint. The width of the HZ is one of the primary factors affecting the formability of the welded joint, and the narrower the hardening zone is, the better the formability of the welded joint is. The effect of laser welding speed on the width of hardening zone is more obvious, and the formability of welded joint can be improved by reducing the width of hardening zone. In this paper, the forming ratio of laser welded joint of 2 mm thick medium Mn steel can reach 0.66.