Graphitic carbon nitride (g-C3N4) has been commonly used as photocatalyst with promising applications in visible-light photocatalytic water-splitting. Rare studies are reported in applying g-C3N4 in polymer solar cells. Here g-C3N4 is applied in bulk heterojunction (BHJ) polymer solar cells (PSCs) for the first time by doping solution-processable g-C3N4 quantum dots (C3N4 QDs) in the active layer, leading to a dramatic efficiency enhancement. Upon C3N4 QDs doping, power conversion efficiencies (PCEs) of the inverted BHJ-PSC devices based on different active layers including poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PC61BM), poly(4,8-bis-alkyloxybenzo(1,2-b:4,5-b') dithiophene-2,6-diylalt-(alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diyl):[6,6]-phenyl C-71-butyric acid methyl ester (PBDTTT-C:PC71 BM), and poly[4,8-bis(5-(2-ethylhexyl)hiophen-2-yl) benzo[1,2-b: 4,5-b'] dithiophene-co-3-fluorothieno [3,4-b]thiophene2- carboxylate] (PTB7-Th):PC71 BM reach 4.23%, 6.36%, and 9.18%, which are enhanced by approximate to 17.5%, 11.6%, and 11.8%, respectively, compared to that of the reference (undoped) devices. The PCE enhancement of the C3N4 QDs doped BHJ-PSC device is found to be primarily attributed to the increase of short-circuit current (J(sc)), and this is confirmed by external quantum efficiency (EQE) measurements. The effects of C3N4 QDs on the surface morphology, optical absorption and photoluminescence (PL) properties of the active layer film as well as the charge transport property of the device are investigated, revealing that the efficiency enhancement of the BHJ-PSC devices upon C3N4 QDs doping is due to the conjunct effects including the improved interfacial contact between the active layer and the hole transport layer due to the increase of the roughness of the active layer film, the facilitated photoinduced electron transfer from the conducting polymer donor to fullerene acceptor, the improved conductivity of the active layer, and the improved charge (hole and electron) transport.