Abstract: This paper studied the on-line monitoring of PWR 3D power distribution based on the harmonics expansion method. In this method, the power distribution is expanded by the neutron diffusion equation’s harmonics. And the expansion coefficients are calculated based on the in-core detector measurements. Non-linear iteration semi-analytic nodal method combined with Krylov sub-space method is used to obtain the harmonics of neutron diffusion equation. This nodal method is 100 times faster than the fine-mesh finite differencing method combined with Krylov sub-space method for harmonics calculation. A 3D power distribution on-line monitoring system named NECP-ONION（On-line monitoring system） has been developed based on the harmonics expansion method. Measurements from a typical PWR core in China are used to validate this system. Numerical results show that the root-mean-square errors of assembly averaged powers are less than 2%. NECP-ONION has high calculation accuracy.