The invention relates to a fabrication method of a capacitor and a fabrication method of a complementary metal-oxide semiconductor (CMOS) image sensor. The fabrication method of the capacitor comprises the following steps of providing a substrate formed with a shallow trench isolation (STI) structure, wherein the upper surface of the STI structure is higher than the surface of the substrate; forming a stack layer on the substrate and the STI structure, wherein the stack layer comprises a stress buffer layer and a hard mask layer on the stress buffer layer; forming an opening exposed out of the surface in the stack layer; forming a protection side wall on the side wall of the opening; etching the substrate along the opening to form grooves; removing the stack layer with a certain thickness; removing the protection side wall, and forming a dielectric layer and a conductive layer, wherein the dielectric layer and the conductive layer cover the surface of the remaining stack layer, and the grooves are filled with the dielectric layer and the conductive layer; and removing the remaining stack layer, the dielectric layer and the conductive layer on the surface of the substrate, wherein the conductive layer and the dielectric layer in the grooves and the substrate form the capacitor. The scheme provided by the invention is used for solving the following problem that electricity between the capacitor and an active region near to the substrate is probably leaked due to poor electrical isolation effect of the STI structure in the traditional fabrication method of the capacitor.