This paper, a fault diagnosis approach for nonlinear dynamic circuit is presented based on harmonic decomposition and coherent measurement. According to the separability of Volterra spectral in weakly nonlinear circuit under AM stimulation, the Volterra response can decompose into linear sub-circuits based on ARMA harmonic decomposition firstly. In linear sub-circuits, the mapping relationship between the fault characteristics and fault states will become more clearly. Then, the dynamic linear sub-circuit is transformed into static circuit by coherent measurement, and the diagnosis equation is created with circuit node-equation. The fault diagnosis can be implemented by calculating the nodes’ fault-current. Finally, with an example to illuminate the proposed is available.