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.