Abstract: Under the background of dual carbons, the coupling characteristics of carbon capture-power to gas (CC-P2G) may affect the safety and reliability of the integrated electricity and gas systems(IEGS) with the gradual application of the carbon capture and storage technology to the coal-fired units. To assess such impacts, a cascading failure assessment of the IEGSs considering the CC-P2G coupling characteristics is proposed, based on the off-design and refined models of the coupling devices. Firstly, the process of power to gas is divided into water electrolysis and methanation, in which the carbon dioxide from the carbon capture devices is used, and the off-design and refined model of the CC-P2G devices is built. The off-design model of the gas turbines is also built based on the refined process of gas to power. Secondly, according to the flow inertia differences between the electric energy and the natural gas, the cascading failure models of the power systems based on the DC steady-state power flow and the natural gas systems based on the dynamic power flow are respectively presented for co-simulating the cascading failures in the IEGSs. Finally, the evaluation indexes in topology and physics are proposed to quantitatively evaluate the impacts of the cascading failures on the power system and the natural gas system. The effectiveness of the proposed method is validated by the integrated electricity and gas test system composed of an IEEE 39-bus power system and a 29-bus natural gas system.