The fluctuating pressure for three plug dissipaters，named as linear plug, stepped plug and gradually contracted plug, were measured and analyzed. From the perspective of fluctuating pressure, the characteristics of anti-cavitation for gradually contracted plugs which were arranged in discharge tunnel were investigated. The results showed that the maximum RMS（root-mean-square） of wall fluctuation pressure appears at 0.1 ~ 0.3 times tunnel diameter away from inlet of the plug, decreases rapidly along the plug wall and then becomes stability after 4 times tunnel diameter. The RMS of wall fluctuation pressure of the contracted plug is smaller than that of other two plugs when the outlet diameter and the flow velocity are certain. The fluctuating pressure distribution is approximately the Gauss distribution. For gradually contracted plug, it is conservative to calculate the minimum pressure using 6 times the RMS of fluctuation pressure. Pressure distribution were measured and analyzed for a plug discharge tunnel on 1∶40 orthodox model. The results showed that the minimum pressure in the tunnel is 3.4×9.8 kPa, so the probability of cavitation occurrence is small.
