In summary, we have studied the effect of chemical environments on the photocatalysis of PFBT Pdots and concluded that the protonation of benzothiadiazole (BT) in PFBT polymer could be an important intermediate process for photocatalysis in acidic conditions. From photocatalytic experiments, the PFBT sample with less than 10 ppm Pd showed comparable photocatalytic activity to the sample with 1000 ppm. It indicates that the active residual Pd amount should be much less than the actual residual Pd in the polymer if Pd is the only catalytic site. With a model compound BTDF in electrochemical experiments, the reaction rate of protonation of BT unit is found to be a fast and diffusion-controlled reaction. The results suggest that the protonated BT likely is an important intermediate of PFBT polymer at acidic conditions, indicating a possible proton-coupled electron transfer (PCET) process from the reduced protonated polymer to the intrinsic catalytic sites. This work, therefore, paves the road to understanding the reaction mechanism of polymeric photocatalysts with heteroatom block units.