As the current standard NB/T 35023—2014 “sluice design specification” involves stilling basin with rectangular cross section, excluding stilling basin with trapezoid section, energy dissipation calculation for stilling basin with trapezoidal cross section were studied on the basis of the classical hydraulics theory and numerical analysis. The analytical formula of shrinking depth and the equation of conjugate depth after hydraulic jump of stilling basin with trapezoidal cross section were derived. Using theory of high-order equation, simple iterative formulas for conjugate depth after hydraulic jump of stilling basin with prismatic trapezoidal cross section and diffused trapezoidal cross section were given, respectively. Finally, the calculation formulas for depth and length of the stilling basin with trapezoidal cross section were obtained according to the energy dissipation equation. Calculation results of a practical project validate the proposed formula has high accuracy.