Abstract: The arc stud welding method was used to weld 45 steel plate and hollow stud with outer diameter of 20 mm and inner diameter of 10 mm. Effects of the rotating arc magnetic field on microstructure and mechanical properties were studied. The results show that with the longitudinal rotating arc magnetic field, the charge in the arc moved spirally under the action of Lorentz force, which extended outward to some extent and the appearance of the joint was improved. The thermal cycle condition of joint was improved thanks to the continuous stirring of molten pool by controlled arc, and the solidification and crystallization of molten pool was affected. The proeutectoid ferrite showed a regular network distribution and the pearlite became fine. For the decrease of joint temperature gradient, the solid-state phase transition time increased, the martensite content in the heat affected zone of the parent metal of 45 steel side decreased significantly and the width of HAZ became smaller. When the rotating arc magnetic field current was too large, the spatter tendency increased obviously as well as the porosity in the weld increased. When the welding time of 1 400 ms, the welding current of 800 A and the rotating arc current of 0.43 A, the arc was fully burned at the full end face of the hollow stud, the shear strength of joints can reach 325 MPa.