Intimately coupled photocatalysis and biodegradation (ICPB) is an emerging technology for treating refractory wastewater. The prerequisite of successful ICPB is that photocatalytic reactive oxygen species (ROSs) attack refractory pollutants and yield biodegradable intermediates, but "instantly" extinct before they transfer to the biofilms. However, photocatalytic induced H2O2 (a typical ROS) is different from others due to its long half-life. Photo-H2O2 in ICPB could potentially damage the biofilm; however, this has not been verified to date. In this study, we show that photo-H2O2 damages biofilms in ICPB, and we successfully addressed this issue using a strategy with the co-substrate (NaAC). Under H2O2 stress, the living cell percentage in ICPB dropped significantly, and the cell morphology presented the disinfection of bacteria cells, with enlarged size and disappeared organelles. By adding NaAC, the living cell percentage increased by 19.6% and the bacterial intracellular structure became integrated. The Shannon index increased from 3.45 to 4.05, indicating enhanced microbial diversity. Thauera and Dechloromonas were dominant genera, which played important roles in the breaking of the aromatic ring and in dechlorinating. As a result, 4-CP removal and mineralization increased by similar to 27% and similar to 23%, respectively. Here, we identified the negative effects of photo-H2O2 on the microbial survival of ICPB and for the first time propose an efficient co-substrate strategy to remedy this issue.