Integrated Energy System (IES) covers a variety of energy such as electricity, heat and gas, which have complex operating states and diverse dynamic characteristics of the equipment. Accurate state estimation (SE) helps IES operate safely and reliably. In this case, a method is proposed for IES mixed state estimation regarding to the poor timeliness and accuracy of IES estimation. Firstly, the influence of the coupling measurement model on the redundancy of IES measurement is analyzed, and the effect of heat network state estimation is improved according to the variation of redundancy. Then, the distributed state estimation is introduced into IES to construct two regional partitioning scenarios on the shared node type and shared line type. Finally, the simulation analysis of IES coupling between a 30-node power network and a 17-node heat network is investigated to verify the effectiveness and practicability of the proposed method.