Abstract: Electrolytic capacitors are one of the component devices that easily fail in a converter. Thus, accurately identifying its degradation parameters, such as capacitance value and equivalent series resistance(ESR), is crucial for improving the reliability of the converters. Taking the single-phase active neutral-point clamped(ANPC) converter as the object, a non-intrusive method for identifying the degradation parameters of electrolytic capacitors of single-phase converter is proposed. The degradation mechanism of the dc-link electrolytic capacitor in a single-phase converter is analyzed, and twin models of the main circuit, sampling circuit, and modulation circuit of the single-phase converter are established. The state equations are discretized using the fourth-order Runge-Kutta method to complete mapping the physical model to the virtual space. Based on the characteristic parameters of aluminum electrolytic capacitor degradation, the state monitoring target function is determined, and the improved particle swarm optimization(PSO) algorithm is used to solve the issues about monitoring the capacitance value and ESR. Finally, a 400 W single-phase ANPC converter prototype is built to verify the effectiveness of the proposed method.