The slit-type energy dissipater (STED) is widely used in hydraulic projects of high water head, large discharge, and narrow river valley, thanks to its simple structure and high efficiency. However, the water wing caused by the shock waves in the contraction section of the STED may bring about harmful effects. A coefficient is introduced for the application of Ippen’s theory in the STED. The expression of the coefficient is experimentally obtained. Simplified formulas to calculate the shock wave angle and the water wing scope are theoretically derived, with relative errors within 5%.