Abstract: In order to explore the influence of the length of the toothed piers on the hydraulic characteristics of the tooth-block inner energy dissipaters, the k-ε mathematical model in the Fluent simulation software is used to numerically simulate it, and the results obtained from the numerical simulation calculation are compared with the results of the previous physical model test, and the accuracy of the numerical model of the internal energy dissipaters with different lengths of toothed piers is verified and proved. After that, the model is used to analyze and study the hydraulic parameters such as central axis flow velocity, time-average pressure, flow coefficient, energy dissipation rate, turbulent kinetic energy and flow field of 7 kinds of pier-type internal energy dissipaters with different pier relative lengths（The relative length η is the ratio of the length l of the toothed piers to the inner diameter D of the simulated pipe）. The numerical simulation results show that the distribution law of the central axis flow velocity of the toothed piers type internal energy dissipaters for the schemes with different relative lengths is basically the same. The maximum flow velocity value appears at the same position. The position of the maximum flow velocity value is fixed and does not change with the change of the relative length of the toothed piers. Under different relative lengths, the position appears 0.38D behind the inlet of the toothed piers. The time-averaged pressure of the front wall and the rear wall of the toothed piers changes obviously, and there is a lower pressure area in these two areas. There is a vortex area in the rear area of the toothed piers, and the turbulent kinetic energy value inside the vortex area is relatively high, and the length of the reflow zone on the rear side of the piers gradually decreases with the increase in the relative length of the piers. Within the simulation range of the numerical simulation, the magnitude of the flow coefficient increases first and then decreases with the increase in the relative length of the toothed piers, and η is within the range of 0.2 to 0.9, the value of the flow coefficient of these relative length schemes is relatively large, and the change law of the value of the energy dissipation rate is opposite to that of the flow coefficient. When η is more than 0.9, the vortex area of the part of constriction expansion on both sides of the toothed piers section separation occurs. After that, as the relative length of the toothed piers increases, the effect of the excess length on energy dissipation is only caused by the head loss along the route, and the excess length does not contribute much to the energy dissipation rate of the energy dissipaters, therefore, the influence of excess length on energy dissipaters can be ignored. Comprehensive analysis shows that the length of the toothed piers with relative lengths in the range of 0.2～0.9 is superior, which provides a basis for the optimization of the body shape of the tooth-block inner energy dissipaters.