The invention belongs to the technical field of nuclear reactors and particularly relates to a fixing device of a control rod guide tube of a nuclear reactor. The fixing device of the control rod guide tube of the nuclear reactor comprises a lower joint and a lower connected socket, wherein the lower joint and the lower connected socket are arranged at the lower end of the control rod guide tube; the lower joint is inserted into the lower connected socket; the lower connected socket is a regular-octagon cylindrical structure, and a guiding groove and a spring hook jack on the same axial line are arranged on four spaced sides at the upper end of the lower connected socket; the lower joint is divided into three sections, the upper section is in a cuboid shape with a cylinder in the middle part, and the middle section is in a regular-octagon cylindrical shape; and the spring hook consists of a spring hook body and a hook body, the spring hook body is provided with a screw hole, and an inclined surface lug is arranged under the hook body. The secure fixation of the control rod guide tube can be realized by the fixing device of the control rod guide tube of the nuclear reactor, and the fixing device of the control rod guide tube of the nuclear reactor has the advantage of convenience for disassembly.