The invention provides a method for demarcating an out-of-pile detector of a nuclear reactor. The method comprises the steps that when the reactor core is in a steady state, measured in-pile axial power distribution and a measured out-of-pile reactor electric current are obtained at the same time through a flux pattern processing software combing a measured value and a theoretical value of an in-pile reactor, and the measured in-pile axial power distribution and the measured out-of-pile reactor electric current are combined to form a first interface document; a calibration coefficient document is generated through an off-line flux pattern processing software; a reactor-core model is predicted through theory, multiple axial power distribution vibration is simulated, amendment is conducted on every time of simulated axial power distribution vibration, and multiple analog interface documents are formed respectively. The first interface document and the multiple analog interface documents are processed through an in-pile calibration coefficient processing software and an out-of-pile calibration coefficient processing software, and the relevant calibration coefficients of the in-pile calibration coefficient processing software and the out-of-pile calibration coefficient processing software are obtained. According to the method for demarcating the out-of-pile detector of the nuclear reactor, a theoretical calculation mode is adopted to replace the actual disturbance of the reactor core, the disturbance of man-made power reduction in a unit is decreased, loss on a measuring hardware system is decreased, and the economical efficiency is improved greatly.