Selective photocatalytic reduction of CO 2 has been regarded as one of the most amazing ways for re-using CO 2 . However, its application is still limited by the low CO 2 conversion efficiency. This work developed a novel Pt/In 2 O 3 /g-C 3 N 4 multifunctional catalyst, which exhibited high activity and selectivity to HCOOH during photocatalytic CO 2 reduction under visible light irradiation owing to the synergistic effect between photocatalyst, thermocatalyst, and heterojunctions. Both In 2 O 3 and g-C 3 N 4 acted as visible photocatalysts, in which porous g-C 3 N 4 facilitated H 2 production from water splitting while the In 2 O 3 nanosheets embedded in g-C 3 N 4 pores favored CO 2 fixation and H adsorption onto the Lewis acid sites. Besides, the In 2 O 3 /g-C 3 N 4 heterojunctions could efficiently inhibit the photoelectron-hole recombination, leading to enhanced quantum efficiency. The Pt could act as a co-catalyst in H 2 production from photocatalytic water splitting and also accelerated electron transfer to inhibit electron-hole recombination and generated a plasma effect. More importantly, the Pt could activate H atoms and CO 2 molecules toward the formation of HCOOH. At normal pressure and room temperature, the TON of HCOOH in CO 2 conversion was 63.1 μmol g -1 h -1 and could reach up to 736.3 μmol g -1 h -1 at 40 atm.