The side contraction of the water behind the gates is more serious and the water flow diffuses longitudinally, resulting in a more complex flow pattern, when the plane gate of the deep-hole pressure tunnel operates with small opening. Therefore, some engineering measures need to be taken to improve the flow pattern behind the gates. Based on the physical model experiment and the theoretical analysis, the hydraulic characteristics behind the gate under different heights of diversion ridge are studied in this paper. The results show that the side contraction of the water behind the gate decreases with the increase of the height of the diversion ridge, the height and length of the tongue of the water behind the gate decrease significantly and the negative pressure of the bottom plate basically disappears, and the pressure distribution tends to be uniform. However, the increase of the height of the diversion ridge in the gate chamber has a certain influence on the flow capacity. Compared with the plane bottom gate chamber, the flow capacity under the gate decreases with the increase of the height of the diversion ridge. In order to avoid affecting the flow capacity, the height of the diversion ridge should not be too high.