Given the significant uncertainty in the sealing effectiveness of water seals for high-head gates caused by material attenuation effects, the attenuation characteristics (including stress relaxation, hysteresis effect, and plastic deformation) and time-varying laws of rubber materials were analyzed through constitutive characteristic tests of rubber water seals. A mechanical model for sealing was established by integrating the sealing mechanism of water seals filling the narrow gap space, and the constraint conditions for reliable sealing of water seals were clarified and experimentally verified. The results indicate that the compression ratio is a key factor affecting the sealing effectiveness of water seals for high-head gates; specifically, the sealing effectiveness of water seals with a compression ratio of less than 25% shows a good positive correlation with the compression ratio, whereas that of water seals with a compression ratio of greater than 25% shows a significant negative correlation with the compression ratio. The established constraint conditions for sealing effectiveness agree well with the ultimate pressure-bearing test results, which can provide an important reference for the reliable sealing of high-head gates.