The Integrated Electricity and Heat System (IEHS) is recognized as an important trend to develop a more clean and efficient energy system. As electricity and heat systems of IEHS can be managed by different energy operators in practice, it is a new challenge that how to coordinate these independent operators under contingencies. This paper proposes a novel risk assessment method for IEHS with independent electricity system operator (ESO) and heat system operator (HSO) based on game theory. Firstly, we present a Stackelberg game model to coordinate the post-contingency dispatch of ESO and HSO, thereby reducing the economic loss of entire IEHS under contingencies. Then, an efficient solution algorithm of the bi-level game model is developed based on Karush-Kuhn-Tucker optimality conditions, strong duality theory and second-order cone relaxation. Furthermore, the novel risk indices are proposed to comprehensively depict the risk of entire IEHS and benefits of each operator, and the risk assessment procedures are developed based on Monte Carlo simulation. Case studies were conducted on an IEHS test case, and numerical results demonstrate the proposed method can effectively reduce both the risk of entire IEHS and the expected cost of operators.