[en] Highlights: • Photocatalytic CO_2 reduction combined with H_2 through water splitting is studied. • The carbon in CO can form CH_3OH directly, so can increase CH_3OH concentration. • Excessive CO will react with HCOOCH_3, resulting in a reduced CH_3OH concentration. • CH_3OH concentration increases with increasing the reactor temperature and pressure. • CH_3OH product and reaction rate vary widely with time due to changing sun light. - 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_3OH 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_3OH directly, however the excessive CO will react with HCOOCH_3 to form CH_3CHO, which results in a reduced CH_3OH concentration. Besides, the CH_3OH concentration subsequently increases as the temperature and pressure increase, and the CH_3OH product and reaction rate vary widely with time due to the changing sun light intensity during the day.