Micro grooves with a semicircular cross-section of tens of microns in radius on metal foils have posed a challenge for machining. This paper proposed stepwise manipulating process parameters in wire electrochemical micromachining (WECMM) to improve the profile accuracy of micro grooves with a semicircular cross-section via evenly distributing the machining gap. Firstly, a two-dimensional model of the electric field was established to investigate the groove shaping process including the changes of machining gap distribution at all normal directions of a cylindrical wire electrode. It indicated that the groove machining process ended before it reached the equilibrium status of WECMM. The corresponding machining gap at the wire electrode feeding direction is much smaller than those at other normal directions. Thereafter, a non-uniform machining gap distribution brings the semicircular profile error when using a cylindrical wire electrode. In this study, stepwise manipulations of the applied voltage and electrode feed rate is proposed to apply at the moment when the machining allowances at all directions are at a comparative range. According to simulations, an optimized stepwise parametric manipulation of increasing the pulse voltage from 12.5 to 14 V and decreasing the wire electrode feed rate from 0.5 to 0.1 μm/s was activated at 113 s and proceeded till 145 s. In this way, the machining gap distribution uniformity is improved and the machining allowances at major normal directions of the cylindrical wire electrode are reduced to zero. Experiments verified that the groove profile error was reduced from $$ {3.39}_{-0.45}^{+0.22} $$ μm to $$ {3.38}_{-0.13}^{+0.16} $$ μm with optimized stepwise-manipulation. Finally, the feasibility of simultaneously machining 20 micro grooves with a semicircular cross-section of 50 μm in radius was verified in experiments.