For studying combustor-isolator matching of dual-mode Scramjet and analyzing the performance requirement of isolator in different mode, based on lumped parameter equations for performance computation of scramjet put forward by our team, critical energy method was put forward for computation of mass flow balance between isolator and combustor, the computation model of Scramjet was built, and the corresponding computer program was developed, the dual-mode Scramjet performance and mass flow balance in different mode were computed and analyzed, one dimension flow parameters of isolator and combustor were gained and study in different mode, and the isolator performance requirement of Scramjet were found. The maximum fuel mass flow and isolator separate length near choking margin was computed when flight Mach number Ma₀=4.0~7.0. It was shown from the computation results that: critical energy method was correct and effective and it could be used for computation of mass flow balance between isolator and combustor, the isolator separating length is much longer when the Scramjet operates in choking mode at high flight Mach number, and it should be took into basic consideration for engineering design.