Abstract As a promising way to control greenhouse gas emission and alleviate global energy shortage, photocatalytic reduction of carbon dioxide attracts more attentions in recent years since it can produce fuels efficiently with the combination of H 2 through water splitting. In this work, a computational model which characterizes the photocatalytic reduction of carbon dioxide by CO co-feed in a novel twin reactor is developed with three subsidiaries of chemical reaction kinetics, gas–liquid mass transfer, and transient sun light intensity distribution. Thanks to previous experimental work as the reliable verification for the numerical simulation, the variations of the CH 3 OH concentration with the CO/CO 2 ratio of gas mixture, pressure and temperature are obtained and analyzed. The results show that the carbon in CO can form CH 3 OH directly, however the excessive CO will react with HCOOCH 3 to form CH 3 CHO, which results in a reduced CH 3 OH concentration. Besides, the CH 3 OH concentration subsequently increases as the temperature and pressure increase, and the CH 3 OH product and reaction rate vary widely with time due to the changing sun light intensity during the day.