The invention provides a fine power reconstruction method for a fuel assembly. The method includes the steps that 13 boundary conditions of fast group and thermal group node volume flux, neutron flux of the four faces of a node, neutron current and neutron flux of four angular points and a node x-direction and y-direction transverse one-dimensional integral neutron flux expansion function are acquired through calculation of a nodal method; a fast group neutron flux distribution expansion function and a thermal group neutron flux distribution expansion function based on an orthogonal function family primary function are acquired; four boundary conditions are added, bothway integration is conducted on the fast group neutron flux distribution expansion function and the thermal group neutron flux distribution expansion function, and neutron flux distribution restrained by a weighting function is acquired; 17-order expansion coefficients of fast group and thermal group neutron even flux distribution are calculated according to the acquired 13 boundary conditions and the neutron flux distribution restrained by the weighting function; fine power distribution of the fuel assembly is obtained accordingly. The calculation precision and calculation efficiency can be remarkably improved, calculation is easy and convenient, and code reconstruction is easy.