A focusing and shielding device for encephalic photon knife consisting of a body of ray source, a switch and an armet, the ray source body is a hemispherical shell with certain thickness, a ray source cavity for placing ray source and a pre-collimation hole are defined on the hemispherical shell; the ray source cavity is defined on the outer surface of the hemispherical shell, and the pre-collimation hole is define on the inner surface of the hemispherical shell and connecting with the ray source cavity; the switch is defined inside the body of the ray source, and the outer surface of the switch is in hemispherical shape, a middle collimation hole is set on the switch; the armet is deposited inside the switch, and the inner surface is in columnar shape, an end collimation hole is defined on the armet, a therapy path is defined where the middle collimation hole connected with the pre-collimation hole to form the end collimation hole. The focusing and shielding device for encephalic photon knife of the present invention is simple in structure and low in cost, and as the inner surface of the armet is in a column shape, it expand the space for therapy.