In the present work, the hererostructured catalysts of g-C3N4/TiO2 were synthesized and well characterized by XRD, SEM, TEM, Raman, UV-vis diffuse reflectance spectra, PL, Mott-Schottky, and XPS. Simultaneous photo reduction of Uranium(VI) and photooxidation of Arsenic(III) was firstly achieved over the g-C3N4/TiO2 catalysts. And the experimental results show that the removal rate of U(VI) decreases with the increase of As(III) concentration, whereas the photooxidation rate of As(III) to As(V) increases with the increase of As(III) concentration. Noteworthily, the photoreduction of U(VI) to U(IV) and photooxidation of As(III) to As(V) was confirmed by XPS analysis in time-scale. The experimental results of free radical capture and quantitative test indicate that holes, hydroxyl radical and superoxide radical are the major active species for photooxidation of As (III), while U(VI) was reduced to U(IV) by the photogenerated electrons. Furthermore, a possible mechanism was proposed to well explain the improved photocatalytic performance of the g-C3N4/TiO2 and the competitive relationship between photoreduction of U(VI) and photooxidation of As(III). The present work develops a heterostructured catalyst for potential application to the simultaneous removal of U(VI) and As(III), and makes clear the effect of photooxidation of As(III) on photoreduction of U(VI) for the first time.