An algorithm for pressure equation in a computational code PCCSAP-3 D,which is dedicated to analyze transient process of PCCS(Passive Containment Cooling System),is introduced.GMRES is introduced to accelerate calculation speed.Pre-conditioning of GMRES is applied.Convergence rates of GMRES and ML-ADI are compared.It shows that pre-conditioned GMRES can be used to solve pressure equations more efficiently.Since pressure equation coefficient matrix is structural and sparse,pre-conditioned GMRES is more efficient than ML-ADI as pressure equation coefficient matrix is larger.