Aiming at the problem of the impact of gas pressure and flow change on the small signal stability of power system for combined electricity and gas system, a small signal stability model of combined electricity and gas system considering the gas transmission delay, dynamic characteristics of natural gas pipeline and gas generator set is established. Firstly, the mathematical relationship of gas pressure changes to relevant eigenvalues and flow changes to relevant eigenvalues is derived under the delay of gas transmission to reveal the mechanism of the influence of gas status on the small signal stability of power system. Then the participation factor method of gas changes related oscillation mode is proposed. Through developing the participation factors of gas changes that affect the relevant oscillating modes, it is possible to quantitatively evaluate the influence of gas flow pressure and flow on the small signal stability of the system.The simulation results of CHP driven by natural gas access infinity system and the improved IEEE 9 node system with two CHPs show that the proposed model and method are correct and effective.