Abstract: Jacobian.free Newton-Krylov(JFNK) method is analyzed and improved by using physical constraints in iteration. Consequently.physical constraints are always satisfied in iteration process of the improved JFNK method.Non。physical phenomenon is avoided.In particular.there is no negative temperature as the method is used for 2-D 3-T energy equations.Robustness of JFNK method is improved.

Key words: JFNK, Newton-Krylov method, iteration method, physical constraint, 2-D 3-T energy equation