[en] An efficient space-time kinetic method, the Improved Quasi-Static Synthetic Nodal Green's Function Method (IQSYN/NGFM), is presented here. In this method, the multidimensional transient equations are changed into two coupled equations by factorizing the neutron flux into shape function and amplitude function. The theory of NGFM is used to solve the time-and space-dependent shape function equations; the PpqM method, which is very effective for stiff equations, is adopted to obtain the solutions of time-dependent amplitude function equations. To enlarge the time step further, the shape function is expanded again, and the undetermined coefficients are obtained by weighted residual approximation