The invention discloses an image sensor and a forming method thereof. The image sensor comprises the following parts: a substrate, which comprises a plurality of mutually separated photoelectric regions; an isolation layer, which is located on the surface of the substrate, wherein the material of the isolation layer has a first refractive index; openings, which are located in the photoelectric region isolation layers, wherein the included angles formed by the side walls of the openings and the top surfaces of the isolation layers are obtuse angles; an optical filter, which is located in the opening, wherein the material of the optical filter has a second refractive index, and the second refractive index is greater than the first refractive index; and a lens, which is positioned on the surface of the optical filter. According to the image sensor, optical crosstalk can be reduced, and meanwhile the photon conversion efficiency can be improved.