Abstract: Differential amplification circuit is an important component of analog electronic technology and a significant form of circuit modeling. This article comprehensively summarizes the relevant knowledge of symmetrical differential circuits, defines the merged-circuit model concept of symmetrical differential circuit based on the construction of superposition state number system-merged-number and its operation rules, solves the key problem of modeling two communal networks, common mode two terminal network and differential mode two terminal network, and proposes a complete analysis and solution method for merged-circuit model of symmetrical differential circuit, in order to present a complete knowledge system of merged-circuit model of differential circuit.