In order to enhance the efficiency of large-scale water decomposition, it is important to understand the oxygen bubble evolution on the electrode surface. In this work, a numerical model for the growth of oxygen bubbles on the electrode surface is proposed based on the dissolved oxygen flux at the bubble boundary, and the mechanisms of the reaction area and current during the bubble growth are investigated. The results show that the bubble diameters calculated from the oxygen flux at the bubble boundary are in good agreement with the diameters of the bubbles growing in the control phase of the chemical reaction. As the reaction region increases, the transition time from the diffusion-controlled stage to the chemical reaction-controlled stage becomes longer during the bubble growth. The concentration maximum value on the microelectrode surface is significantly higher than that on the large electrode surface, which leads to a steeper concentration gradient between the microelectrode surface and the bubble surface. As the current increases, the bubble growth rate increases and the time coefficient decreases faster. The bubble diameter at a current of 0.06 mA accords well with the bubble diameter at a current of 0.1 mA in the photoelectrochemical water splitting experiments. This is because the scattering of light by the growing bubbles leads to a decrease in the current density at the bottom of the bubble.