The invention relates to a manufacturing method of an organic solar cell. First deposit in order a first electrode and a first transmission layer on a substrate. Then coat a photoresist layer having a preferred thickness ranging from 1000 nm to 1600 nm on the surface of the first transmission layer by a spin coater at a preferred spin-coating speed ranging from 3000 rpm and 6000 rpm for 30 seconds. Then develop an area by a photolithograph process, and coat an organic active layer in the area and on the surface of the photoresist layer by use of the spin coater at a preferred spin-coating speed ranging from 500 rpm and 800 rpm, in which the organic active layer has a thickness ranging from 230 nm to 320 nm at 500 rpm. Final deposit a second transmission layer and a second electrode on the organic active layer to obtain the organic solar cell.