The invention discloses a reactor nuclear power monitoring method and system. The method comprises A, after the reactor power level is stable, measuring neutron noise signals through two symmetrically arranged neutron gamma compensation ionization chambers, B, calculating time signal FFT through the two neutron noise signals, calculating cross-spectral density phi XY and carrying out normalization, C, fitting the cross-spectral density and calculating cross-spectral density phi XY (omega 1) at frequency omega 1, and D, according to the fitted parameters, acquiring a reactor fission rate F and reactor absolute power P. The method and system can realize parameter measurement without interference on normal operation of the reactor, is suitable for all reactors and realizes fast and convenient measurement.