To solve the three-dimensional transient problems,the Improved Quasi Static Method(IQS) is adopted to deal with the temporal problem,and an improvement of synthesis method(YN) is also introduced.By factorizing the neutron flux,the time step of this method is enlarged so as to decrease the number of spatial calculations.For spatial calculations,Nodal Green's Function Method(NGFM) is used to determine the distribution of shape function,so the spatial mesh of IQSYN/NGFM can be about twenty times as large as that of finite difference method.