Magnetically assisted resistance spot welding (MA-RSW), which uses a pair of mutually repulsive ring-shaped axially magnetized magnets, has been demonstrated to have significant effect on weld quality of high strength steels and light metals. However, in manufacturing of metro car bodies, single-sided resistance spot welding processes is frequently adopted to achieve better surface quality, which poses a big challenge to the traditional double-sided MA-RSW apparatus. In this study, single-sided MA-RSW apparatus with four arc-shaped radially magnetized permanent magnets was developed to adapt the demand of the single-sided RSW process, and 1.5 + 2.5 mm austenitic stainless steels (ASSs) are used to evaluate its effectiveness in terms of weld quality and surface indentation. Meanwhile, a 3D finite element model taking three types of MA-RSW apparatus into account was created to reveal the improvement mechanism of the single-sided MA-RSW apparatus. The results showed that the novel single-sided MA-RSW apparatus can produce stronger radial magnetic field in effective welding region and thus provide a great potential and promotion effect to the weld quality in terms of macro-morphology, surface indentation, microstructure, microhardness, and mechanical properties.