Grooves are used widely in aeroengines, die and moulds and other industrial contexts. Electrochemical single-pass milling (ECSPM) is a flexible method to manufacture high-aspect-ratio grooves using a simple tube tool. However, there is always a taper on the sidewall of the groove due to the dominant flow field and electric field effects in the inter-electrode gap (IEG), which reduces the machining quality. This paper develops a coupled multi-physics three-dimensional model (including turbulent flow field, electric field, heat transfer of fluid and moving mesh) to reveal the formation mechanism for these tapered sidewalls of grooves. Furthermore, a novel tube tool with a wedged end face and partial insulated layer on the outer wall (W-tube) is designed to reduce the sidewall taper by controlling the coupling of the pulsating flow and electric fields. Simulation results show that the low electrolyte flow velocity through the side IEG weakens the removal of electrolytic by-products and leads to non-uniform material removal along the groove depth, thereby forming tapered sidewalls. By using the W-tube tool, the flow and electric fields are coupled in the pulsating state, which eliminates the zones of persistent low flow velocity and disperses the material dissolution process. Also, the coupling phase difference can be controlled by adjusting the position of the insulated layer. Compared with other coupling processes, ECSPM with a coupling phase difference of 0° (type A) is more suitable for enhanced mass transfer, and the difference in dissolution overcut along the groove depth decreases to 0.015 μm in one rotation of the tube tool, thereby reducing the sidewall taper. Experimental results verify the theoretical analysis: the least-tapered groove was that manufactured with pulsating fields coupled exactly in phase, which was realized with the type-A W-tube tool; the taper was reduced to 1.21±0.10° compared to that of 2.21±0.37° with traditional ECSPM, and the surface quality was also improved.