In　order　to　reduce　the　degree　of　freedom　of　finite　element　models　to　make　it　consistent　with　corresponding　experimental　degree　of　freedom,　based　on　the　modified　system　eigen-equation,　an　iterative　dynamic　condensation　formula,　which　reflects　the　transforming　relation　between　system＇s　major　and　minor　degrees　of　freedom　has　been　deduced.　Through　numerical　calculation　and　comparison　with　the　existed　iterative　dynamic　condensation　formulas,　the　formula＇s　convergence　rate　is　quicker　with　better　stability　and　higher　accuracy.