We designed an S-heterojunction system with a perovskite nanocrystal, Cs1–x FA x PbBr3 (CF), coupled with a bismuth oxyiodide (BiOI) nanosheet to form a perovskite heterojunction (PHJ) photocatalyst. On the basis of femtosecond transient absorption measurements, the pristine CF sample has two charge recombination periods, 100 and 900 ps, corresponding to surface and bulk trap-state relaxations, respectively. When CF was in contact with BiOI to form an S-heterojunction, rapid interfacial charge recombination occurred to show two decay components with time coefficients 1 and 35 ps, responsible for the electron–hole recombination in the surface and bulk states, respectively. We observed a new photoinduced absorption band on the blue side of the photobleach band of PHJ that gives relaxation more rapid than that of pristine CF, presumably due to doping of bismuth cations creating defect states to enhance the charge recombination that leads to photocatalytic performance for the PHJ catalyst poorer than for the pristine CF sample.