The purpose of this study is to investigate factors influencing the quality of planar magnetic abrasive finishing under a variable magnetic field in permanent magnetism. It aims to enhance the magnetic field variation in the processing zone, diversify the processing trajectory of magnetic abrasive particles, and improve the efficiency and effects of the magnetic abrasive finishing. The method was designed to replace the constant magnetic field source in traditional plane magnetic abrasive finishing with a variable magnetic field generated by permanent magnetism. Magnetic field simulation software was used to simulate the permanent magnet variable magnetic field, and the distribution of the magnetic field under different conditions was analyzed. Experimental parameters were selected based on the simulation results. A ring-shaped magnet that generated a radial magnetic field was used as the source of the magnetic field. A well-conductive pure iron material was used as the magnetic structural frames to facilitate the variation of the magnetic field through the opening and closing of the magnetic circuit. A stepper motor and a pulse generator were employed to regulate the magnetic abrasive finishing magnetic field, thereby adjusting the alternating frequency of the magnetic poles. The finishing effects on SUS304 stainless steel under different parameters were compared through experiments. Under experimental conditions with a finishing gap of 1.5 mm, the impacts of different finishing times, different abrasive particle sizes, and different motor spindle speeds on the surface quality of the workpiece were compared. The surface roughness measuring instrument and the ultra-depth 3D electron microscope were used to detect the surface quality of the workpiece before and after comparison experiments, and the simulation results were verified. The simulation diagrams of different magnetic field distribution states under permanent magnet variable magnetic fields were compared. The field intensity in the magnetic abrasive finishing area of the permanent magnet variable magnetic field changed significantly, and the magnetic induction intensity was high when the field strength peaked. The magnetic abrasive particles moved periodically in response to changes in centrifugal force and the states of the permanent magnet variable magnetic poles, thereby complicating the grinding trajectory. Under the conditions of a finishing time of 40 min, spindle speed of 245 r/min, and magnetic abrasive particle size of 60 mesh, the surface roughness of the SUS304 stainless steel plate decreased from the original 0.312 μm to 0.060 μm after finishing. In conclusion, the permanent magnet alternating magnetic field not only increases the magnetic induction intensity in the processing zone but also induces periodic movement of magnetic abrasive particles in the processing area, which leads to a complex grinding trajectory. This promotes the renewal of abrasive particles. Compared with the constant magnetic field magnetic particle grinding process, the permanent magnet alternating magnetic field improves the grinding efficiency and effects.