The adsorption of reactant gas molecules on photocatalysts displays an important role in the photocatalytic activity. Herein, the adsorption of CO2, O-2, NO and CO on s-triazine-based g-C3N4 was investigated by density functional theory calculation. Various adsorption sites were designed, and the calculated adsorption energy (E-ads) showed that all these gas molecules preferred to adsorb at the open-hollow site. Moreover, the adsorption energy of the most preferential adsorption system for each gas molecule followed the order of E-ads (CO2) < E-ads (O-2) < E-ads (NO) < E-ads (CO), indicating that the adsorption affinity of these gas molecules on g-C3N4 followed the order of CO2 > O-2 > NO > CO. The electronic properties of the most preferential adsorption systems for these gas molecules were investigated, including band structure, work function, density of states, the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The results in this work sufficiently disclose the adsorption behavior of these gas molecules on s-triazine-based g-C3N4.