To improve the utilization of energy, separate electric power, gas and thermal systems are gradually transformed into an Integrated Energy System (IES). However, with the increasingly complex coupling relationship and operational mode in an IES, each subsystem in the IES faces new requirements for secure operation. First, the steady power flow equations of independent electricity, gas and thermal networks are formulated and integrated into a uniform equation of an IES. Based on the Jacobian matrix of multi energy flow and analysis of the coupling relationship among variables, the comprehensive matrix of sensitivities of voltage, air pressure, branch current, node temperature to the control variables in the IES is calculated. The relationship between control variables and state variables of different energy categories in the IES is described quantitatively. Then, based on the comprehensive sensitivity matrix, the parameters that are the most sensitive to the unsafe points in the IES are adjusted to meet the requirements for the safe operation of the system. Finally, an IES39-26-44 example is used to analyze and verify the proposed method and the simulation results verify the correctness and effectiveness of the proposed method. This work is supported by the National Natural Science Foundation of China (No. 51877181) and the Science and Technology Project of State Grid Shandong Electric Power Company (No. 520611190002).