Photocatalytic N2 reduction is a thermodynamically and kinetically challenging reaction. Many research works have focused on the construction of oxygen vacancy with abundant localized electrons in the photocatalyst to activate N2 and weaken the NN triple bond. In this work, we initially intended to construct oxygen vacancy on Bi2MO6 (M = W, Mo) nanomaterials to achieve improved photocatalytic N2 reduction activity. However, NH4 + determination by Nessler’s reagent method and the ion chromatography method showed completely opposite results, indicating that there were many uncertainties in the determination of the actual NH4 + yield in the photocatalytic N2 reduction experiment. To further systematically clarify the uncertainties in NH4 + determination, different bismuth-related photocatalysts and P25 were also used to investigate the influence factors on photocatalytic N2 reduction. It demonstrated that the residual alcohols, amines, and other N-containing surfactants in the reaction system could significantly interfere with the determination of the actual NH4 + yield. Therefore, we should be cautious of the results of the photocatalytic N2 reduction experiment. It is quite essential to select an appropriate NH4 + determination method and design fully reasonable control experiments to obtain accurate data of photocatalytic N2 reduction.