Abstract: In order to address the issue of large computational workload in the process of optimum loading pattern search for commercial PWR NPP, a new method--Hybrid Harmonics and Linear Perturbation （HHLP）is proposed to perform the fast loading pattern evaluation. The method is characterized by separating the local perturbation effect of neutron flux distribution induced by fuel shuffling from the global flux tilting effect, and using low order harmonics and linear perturbation base function expansion to represent these two effects respectively. Since by adoption of the weighted-residual method, the solution of the original large-scale eigenvalue problem can be replaced by the solution of a small-scale eigenvalue problem on expansion coefficients, the proposed method can significantly reduce the computational time for a loading pattern evaluation. This paper gives an introduction of the theory of HHLP and another accompanying paper will provide examples for various kinds of numerical verification of using HHLP.