Electrochemical trepanning is a highly effective and economic manufacturing technology and often used to process ruled surface parts in the aerospace field. Stray corrosion in electrochemical trepanning causes poor processing localization, large tip taper, and low machining accuracy. This paper presents an innovative approach to the reduction of stray corrosion in electrochemical trepanning in which ceramic particles are used to form an insulating protective layer on the machined surface and thereby inhibit harmful stray currents. The electric field distribution and contour changes during the machining process are simulated using the finite element method in COMSOL software. The results suggest that the presence of the insulating particles can dramatically improve the electric field distribution and reduce stray currents and the taper angle. A series of experiments are performed to verify the simulation results. The experiments show that the particles have a significant effect on the taper angle, reducing it from 4.03° to 2.00° at the same machining speed of 1.0 mm/min. Further experimental results show that the size and quantity of insulating particles both affect surface quality. When the particle diameter is reduced from 2.5 to 1.5 mm, the surface roughness Ra decreases from 1.32 to 1.16 μm. As the particle quantity varies from 250 to 900, a minimum value of Ra = 0.86 μm is obtained for 650 particles.