A method for acquiring fine distribution of the reactor core three dimensional neutron flux density of a reactor comprises the following steps: 1, carrying out geometric modeling to a reactor core, dividing computational domain, dispersing angle space, generating characteristic line, appointing materials for all computing regions, obtaining macroscopic section parameters and setting initial values for neutron flux density flux of a computed region, terminal condition of reactor and characteristic value; 2, calculating coefficient matrix required by the matrix method of each sub region and a part which is positioned at the right end and cannot be changed with the iterative computation; 3, seeking the density of thicknet neutron flux and correcting the density of one-dimensional and two dimensional thinnet neutron flux; 4, seeking the density of one-dimensional neutron flux of each lattice cell; 5, iterative solution of the density of the two dimensional thinnet neutron flux of each layer; and 6, updating three dimensional thicknet parameters, judging whether a characteristic value and the density of a three dimensional neutron flux are in convergence or not, if in convergence, turning to the step 3 for continuous iteration under convergence is achieved, namely, fine distribution of three dimensional neutron flux density can be obtained; and the fine distribution of the reactor core three dimensional neutron flux density of a reactor can be obtained quickly.