Regulating the surface properties of catalysts to control the selectivity of a reaction is a fascinating approach. Bulk polymeric carbon nitride shows poor N.-Phenylhydroxylamine yield in the nitrobenzene reduction reaction mainly due to the uncontrollable condensation side reactions. Therefore, adjusting the structure of the catalyst has been key to solving the above problem. Here, OH group-modified polymeric carbon nitride was produced above a simple hydrothermal treatment. The introduced OH groups replace the terminal amino groups (-NH2) on the surface of the polymeric carbon nitride, a three-fold increase in the reaction rate was achieved together with a high selectivity towards N.-Phenylhydroxylamine (Approx. 80%). The introduced OH group was found to be advantageous for the adsorption of the nitrobenzene on the basis of the calculation of the density functional theory (DFT). It could also decrease the recombination rate of photoinduced electron-hole pairs, which would accelerate the photocatalytic oxidation of isopropanol and provide more protons to participate in the hydrogen transfer process. In addition, the elevated position of the conduction band after the OH modification would provide high energy photogenerated electrons to promote the reduction of nitrobenzene. All of this is important to ensure the highly selective production of N.-Phenylhydroxylamine. This paper not only offers a simple and environmentally friendly approach to the modification of polymeric carbon nitride into an efficient photocatalyst, but also sheds light on the further investigation of selective hydrogenation.