Abstract: An improved quasi-static method was proposed to solve the space-time neutron kinetics equation. Under the improved quasi-static approximation condition, the neutron flux density was decomposed into the range-function and shape-function by the factorization method. Then the approximate analytical solutions in the axial and radial directions were obtained using the eigenfunction method to solve the shape-function. In the solving process, the range-function was simplified as the form of point reactor model, and the analytical solution of three-dimensional space-time neutron kinetic equation for reactor was derived. Compared with other analytical method and numerical solution, the improved quasi-static eigenfunction method has wider application scope and faster computation speed.