The electrothermal anti-icing and deicing system is often used to ensure flight safety and aerodynamic performance when aircraft encounters icing conditions. This paper establishes a coupled calculation model of multi-layer heat conduction and water film flow by using the conjugate coupling method， and develops a series of numerical simulation calculations for the electrothermal anti-icing and deicing process. Myers water film model is able to stimulate non-stationary calculations， while the internal thermal conductivity model of the electrothermal component is implicitly discretized by using the finite volume method. The water film model and thermal conductivity model are loosely coupled through the conjugate heat transfer method by exchanging boundary values. Research has found that during the process of electrothermal anti-icing， the phenomenon of runback water refreezing occurs due to the melting of ice accumulation and the flow of water film. A flow field calculation and aerodynamic analysis are conducted on the icing shape of the electrothermal system in the later stage. It shows that the frozen overflow water on the upper and lower wing surfaces of the wing will cause disturbance to the flow field. The Q-criterion is used to determine the vortex structure behind the icing. Compared with clean airfoils without icing， the freezing of runback water at the leading edge of the wing causes significant oscillations in the pressure coefficient curve. Therefore， the problem of refreezing runback water affects the aerodynamic performance of the wing， which leads to the undesired protective effect of the electrothermal system.