The work aims to design a low-frequency alternating magnetic field generator to remove the inner surface defects of H62 brass tube with high efficiency, so as to improve its surface quality and prolong its service life. A low-frequency magnetic field generator was designed with closed iron core. EDEM software was used to simulate the motion of magnetic abrasive particles in the magnetic field. The magnetic field generated under different winding modes and energization modes were simulated and analyzed with Ansys software. The pipe fitting was clamped and rotated by the NC lathe in combination with the magnetic field generator. The magnetic abrasive particles in the pipe fitting were magnetized in the magnetic field and close to the inner surface of the pipe fitting for grinding and polishing. The test samples before and after grinding were detected and analyzed by stylus surface roughness measuring instrument and super depth of field 3D electron microscope. Based on the three end winding mode, two different energization modes were used to grind the pipe fittings. After grinding for 6 min by the relative energization mode, the roughness of the inner surface of H62 brass tube changed from the original Ra 0.618 μm down to Ra 0.373 μm. After grinding for 6 minutes by the adjacent energization mode, the roughness of the inner surface of H62 brass tube changed from the original Ra 0.667 μm after down to Ra 0.081 μm. The use of low-frequency alternating magnetic field can realize the cyclic renewal of grinding tools (magnetic abrasive particles) and improve the utilization rate of magnetic abrasive particles. When the iron core is energized by three end winding, the magnetic field generated by the electromagnetic device is stronger and more suitable for magnetic abrasive finishing. When the adjacent energization mode (N-N-S-S magnetic pole arrangement) is adopted, the grinding pressure is larger. It greatly shortens the processing time and removes the original defects on the inner wall of the pipe fittings.