The invention provides a method for fabricating CMOS image sensor, including forming a grid structure on a substrate; using a first mask to implement a first ion injection process to form a photodiode on the substrate adjacent to the grid structure; forming a first patterned blocking layer on the photodiode; forming a separator on the side wall of the grid structure and the side wall of the firstpatterned blocking layer; and using the mask pattern of a second mask and the separator to implement the second ion injection process.