The invention relates to the technical field of a nuclear instrument system of a mega-kilowatt class nuclear power plant and provides a method for calibrating the axial power deviation of an out-of-pile nuclear instrument system of a nuclear power plant. The method comprises in the reactor circulation period, carrying out a xenon oscillation test (for acquiring one-step whole-core fluxgraph data and at least four set of partial fluxgraph data) to acquire power range calibration factors alpha for adjusting reactor core axial power offsets of measurement channels of the out-of-pile nuclear instrument system in the power range, utilizing the factor alpha of the last xenon oscillation test in all the cycles from the current xenon oscillation test to the next xenon oscillation test, and updating the parameters KU and KL of each measurement channel according to the factor alpha. The method reduces the frequency of the partial fluxgraph measurement, reduces the disturbance of the xenon oscillation test on a nuclear power unit and reduces the probability of the shutting-down in advance of the nuclear power unit.