On the basis of the traditional bulk heterojunction structure, we introduce a method of gradient doping in the mixing layer to improve photoelectric conversion efficiency (ηp) of organic photovoltaic device (OPV). One of the devices with the structure of ITO/CuPc (10 nm)/CuPc:C60 (2:1) (4 nm)/CuPc:C60 (1.5:1) (4 nm)/CuPc:C60 (1:1) (4 nm)/CuPc:C60 (1:1.5) (4 nm)/CuPc:C60 (1:2) (4 nm)/C60 (30 nm)/Bphen (8 nm)/Al (100 nm) show the improvement on performance:short-circuit current JSC = 9.18 mA/cm2, photoelectric conversion efficiency ηp = 1.35% under AM1.5 solar illumination. Compared with ηp of the traditional bulk heterojunction OPV ITO/CuPc(10 nm)/CuPc:C60 (1:1) (20 nm)/C60(30 nm)/Bphen(8 nm)/Al(100 nm), the ηp of the present structure is improved by 25%, which is attributed to the improvement on percolating paths of donor and acceptor molecules across the entire mixing layer, thus increasing the charge collection efficiency. Consequently, the overall device series resistance is reduced and the photoelectric conversion efficiency is enhanced.