Abstract: On the bases of Stoner's method, the writer advances a new concept of element's state-gradient correlation coefficient β_j, and the equivalent modified pressure drop △P_j.
∂Q_j/∂P_Dj=β_j∂Q_j/∂P_uj
△P_j=△P_uj-β_j△P_Dj
where: P_uj——upstream node pressure of element j, P_Dj——downstream node pressure of element j, Q_j——flow rate through element j. In order to provide a useful and convenient mathematical model for system atic analysis and research, the writer has applied. Kirohhoff's first law and established matrix equations and modified equations for the natural gas transmission system consisting of various elements and including subsyatems for gas exploitation, transmission and distribution and for high, medium and low pressure.