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 CH3OH concentration with the CO/CO2 ratio of gas mixture, pressure and temperature are obtained and analyzed. The results show that the carbon in CO can form CH3OH directly, however the excessive CO will react with HCOOCH3 to form CH3CHO, which results in a reduced CH3OH concentration. Besides, the CH3OH concentration subsequently increases as the temperature and pressure increase, and the CH3OH product and reaction rate vary widely with time due to the changing sun light intensity during the day. (C) 2016 Elsevier Ltd. All rights reserved.