Abstract: The current paper analyses the convergence of SIMPLE type methods by using fourier transform with some assumptions.From theoretical viewpoint,it studies the features of several relaxtion factors,such as time step consistent factor E,pressure relaxtion factor α_p and shows that even if in the linear situation,an unsuitable alternative of relaxtion factors will induces divergence of calculation,and E may play the rde of descreasing time step.Finally,it also compares the convergence rate of SIMPLE with SIMPLER.