Photocatalytic reduction of CO2 into value-added chemicals is particularly attractive as it could produce renewable energy and capture greenhouse gas. Photoreduction of CO2 can be realized over molecular and inorganic catalysts. The former usually exhibit high activity, but low stability and often inactive under visible-light irradiation; the latter has low activity, but good stability. Here we use g-C3N4 nanosheets as the photosensitizer to integrate with Fe tetra(4-carboxylphenyl)porphyrin. chloride (FeTCPP) molecular catalyst. Besides pi-pi stacking between tri-s-triazine unit and porphyrin, the carboxyl group modified Fe porphyrin is used for the first time in CO2 photoreduction so as to form hydrogen bonding with the rich amino groups in g-C3N4 nanosheets. g-C3N4/FeTCPP heterogeneous catalysts are prepared via a facile self-assembly approach, in which light harvest is separated from catalysis spatially and temporally. The obtained g-C3N4/FeTCPP heterogeneous catalysts exhibit high activity for CO2 reduction under visible-light irradiation, with CO yield of 6.52 mmol g(-1) in 6 h and selectivity up to 98%. Fluorescence data indicate that the electrons can efficiently transfer from the g-C3N4 nanosheets to FeTCPP. The mechanism for CO2 reduction over the g-C3N4/FeTCPP heterogeneous catalysts is proposed based on the results of quasi in-situ ESR and UV-vis measurements. This work may pave a facile approach for fabricating the high-efficient photocatalysts for CO2 reduction, as well as better understanding the related mechanism.