One critical issue to which special attention should be paid during the activity evaluation of photocatalytic CO2 reduction is the possible carbonaceous residues on the photocatalyst, which may decompose into CO/CH4, causing overestimation of the activity. In this regard, a contrast test under N-2 atmosphere instead of CO2 has been widely employed as one cost-effective approach to confirm whether carbonaceous residues contribute to the carbon-containing products formation. However, this method might otherwise result in underestimation of the activity, according to the case study of as-fabricated Bi2WO6-TiO2 binanosheets (B-T) in this work. Based on integrative studies of B-T sample under N-2 and CO2 atmosphere, we for the first time unveil the co-existence of a competition and an interaction relationship between light-driven carbonaceous residues decomposition and photocatalytic CO2 reduction, which further emphasizes the necessity of removing organic residues from photocatalysts and carefully analysing the origin of carbon-containing products to estimate the photocatalytic performance towards CO2 reduction in a more accurate way. Moreover, this work could provide some enlightenment on designing and/or synthesising more efficient photocatalysts for CO2 reduction with high selectivity for CH4 formation, where rational construction of Z-scheme heterostructures is highlighted.