Magnesium (Mg) alloys have been considered as an important material in aerospace, sport, electrical, railway systems and, most importantly, in automotive industry, which comprises 80% of Mg usage. In order to enlarge its usage, appropriate welding methods should be investigated. Refill friction stir spot welding (refill FSSW) is a novel solid-state welding technology. Due to the significant merits of keyhole-free weld, improved mechanical properties, etc., it has been successfully used for the welding of Al-based similar and dissimilar combinations. However, studies concerned with Mg alloys are limited. The feasibility of refill FSSW of casting Mg alloy AM50 was investigated firstly. Defect-free welds with flat surface and high performance were obtained successfully. Moreover, a modified method, differential rotation refill FSSW (DR-refill FSSW), was developed. DR-refill FSSW provides a new and effective strategy for improving the performance of spot welds based on microstructural engineering.