Physical and mathematical models are established to simulate the phenomenon of single droplet impinging onto flat liquid film by using the coupled level set and volume of fluid method, and influences of viscosity and surface tension on the morphology of the crown liquid sheet are discussed. Besides, the mechanism of jet flow in the neck region is revealed and the theory of kinematic discontinuity proposed by Yarin and Weiss is verified by analyzing the distributions of pressure and velocity after impingement. The research result shows that the effect of surface tension on the crown morphology is far stronger than that of viscosity. Generation of jet flow is due to the higher pressure difference, the effect of which becomes weak with impact evolution and the jet flow is developed into the crown gradually with the help of radial flow in the liquid film.