Now the cascaded H-bridge is widely used in the field of the static reactive power compensation and the photovoltaic grid connected inverter due to its unique topology, and its influence on power system is becoming more and more obvious. With the increase of power, the switching frequency decreases and the number of cascaded H-bridge increases, which leads to the increase of sampling delay and carrier phase shift delay in digital control. However, the delay will affect the output impedance of cascaded H-bridge, which affects the stability of cascaded H-bridge itself. Aiming at the influence of delay on the stability of cascaded H-bridge, this paper presents a modeling method of cascaded H-bridge sequence impedance considering sampling and carrier phase shift delay. This method deduces the functional relationship between the sampling, carrier phase shift delay and the output voltage of cascaded H-bridge arm side. Based on the switching model of three-phase N cascaded H-bridge circuit, the sequence impedance model of cascaded H-bridge converter is established, which can quantitatively reflect the influence of delay on the output impedance of cascaded H-bridge. Finally, the Simulink simulation platform of the three-phase cascaded H-bridge converter is constructed, and the sweep frequency verifies the theoretical analysis.