The invention discloses an improved method for calculating three-dimensional neutron flux density fine distribution of a reactor core. The method comprises the following steps that 1, modeling is conducted on the reactor core, needed parameters are calculated, and variables are initialized; 2, a radial leakage item is unfolded in the axial direction through a Legendre polynomial expansion, a one-dimensional discrete ordinate difference equation is calculated according to information obtained in the step 1, and an axial leakage item is acquired; 3, two-dimensional spatial distribution of the axial leakage item is obtained through calculation, two-dimensional transportation calculation is conducted according to the information obtained in the step 1, and neutron flux density distribution, a cell-homogenized cross section and the radial leakage item are acquired; 4, whether a characteristic value and the three-dimensional neutron flux density are convergent or not is judged, if not, iteration is continuously conducted from the step to the step 2 till the problems are convergent, and then three-dimensional neutron flux density fine distribution is obtained. According to the method, Legendre polynomial expansion unfolding is conducted on the radial leakage item in the axial direction, the calculation precision is improved, and two-dimensional spatial distribution of the axial leakage item is calculated, so that calculation is closer to true conditions.