On mutually coupling lines, the transmission signal is dispersively propagated by crosstalk coupling between lines and shows complex propagation characteristics caused by reciprocal reflections. Usually, the differential equation and the integral equation have been applied to analyze the solutions of transmission lines. In this paper, we propose a different analytical method of the propagation characteristics of signal and crosstalk noise. By setting up crosstalk coupling line as a sectionally divided digital transmission network and by using the signal flow graph and the difference equation, the propagation characteristics in the frequency domain, the space domain and the time domain on mutually coupling lines can be obtained. To verify the validity of this method and analyze the complex propagation problems, we first study the crosstalk characteristics of a twisted pair cable via the third circuit by unidirectional coupling. Subsequently we will analyze the coupling theory of bidirectional coupling lines.