In this study, g-C3N4/PDI/Fe (gCPF) composite material was prepared by incorporating Fe ion on the composite catalyst of g-C3N4/PDI (gCP). X-ray photoelectron spectroscopy (XPS) showed that the Fe was successfully incorporated on the pristine g-C3N4/PDI. UV-vis diffuse reflectance spectrometry (UV-vis DRS) and Photoluminescence spectral (PL) analysis confirmed the enhancement of the visible absorption band following a decline in the photoelectron/hole recombination rate with gCPF. A preparatory experiment was performed on photocatalytic degradation of p-nitrophenol (PNP) to examine the activity of gCPF. Results obtained in the radical quenching and the electron paramagnetic resonance (EPR) spectroscopic studies indicated that an in-situ Fenton-like system has been successfully established and the main reactive oxygen species (ROS) changed from O-2(center dot)- to both O-2(center dot)- and (OH)-O-center dot in the gCPF system. However, a competition toward conduction band electrons between Fe3+ and O-2 caused an inhibitory effect on PNP degradation. To overcome the effect, nitrilotriacetic acid (NTA) was introduced as a reducing agent for Fe3+. Upon adding NTA, the efficiency of PNP degradation greatly enhanced from 33 to 80%. The effect of initial pH, dosage of NTA and content of dissolved O-2 on PNP degradation was also studied. The photocatalytic stability was confirmed by recycling experiments.