Gas-liquid two-phase flow and mass transfer phenomena widely exist in nature and daily life. Studying the mass transfer process of bubbles in water is of great significance for understanding the mass transfer mechanism and exploring the enhancement of the mass transfer process. The volume of fluid (VOF) method was used to simulate the rising process of CO2 bubble in still water, and the mass transfer process was considered by a user defined function program. The instantaneous velocity, mass transfer coefficient, CO2 dissolved amount, and mass transfer wake changes of bubbles with different initial diameters during the rising process were studied. In the process of bubble rising, the transverse velocity changed periodically and the oscillation amplitude decreased with the increase of bubble initial diameter, while the longitudinal velocity increased with the increase of the bubble initial diameter. Between 3.5 mm and 6 mm, with the increase of the bubble diameter, the CO2 dissolved amount increased, and the wake stream showed three states: symmetrical state, transitional state, and periodic shedding. The critical Re number of 3.5~6 mm bubble wake transition increased with the increase of bubble initial diameter, the frequency of wake periodic shedding was 17~22 Hz, and the departure frequency decreased with the increase of bubble initial diameter. The bubble wake was consistent with the mass transfer wake. With the increase of the bubble initial diameter, the influence range of the bubble mass transfer wake increased.