This study investigates the effects of an external alternating electromagnetic field on the joint formation, ferrite-austenite microstructure, mechanical properties, and corrosion resistance of SUS316L austenite stainless steel narrow-gap laser hot-wire feeding welded joint. The experimental results show that the controlled melting of the filler wire in a transverse direction is achieved by the alternating electromagnetic field. The joints with wider width, shallower penetration and highly symmetric weld are obtained, in addition, non-fusion and pore defects in the joints are also suppressed. The external alternating electromagnetic field introduces vibration energy into the weld pool, thereby changing the original natural convection mode of the weld pool and promoting a high-temperature liquid metal flow from the weld center to the groove sidewall. The stirring effect of the molten pool makes the temperature uniform, decreases the temperature gradient of the molten pool, inhibits the growth of coarser austenite columnar crystals, and increases the grain orientation diversity. The electromagnetic field-assisted welds have small grain size and dendritic spacing as well as a complex dendrite morphology. In summary, the joint tensile and electrochemical corrosion tests show that the electromagnetic field-assisted SUS316L stainless steel narrow-gap welded joint has good mechanical properties and corrosion resistance.