The critical hydraulic gradient of tailing silt is greater than that of conventional sand soil, which is determined by the tailing silt’s structure characteristic,and the calculated critical hydraulic gradient of piping-typed tailing silt by conventional formula is far less than the measured values. Therefore, the conventional calculation method can not be applied to tailing silt. Through collecting large amount of physical indexes and particles characteristic indexes of the tailing silt and comparing the grain size distribution characteristics of flowing soil-typed tailing silt with that of piping-typed tailing silt, we deduced the tailing silt’s grain size distribution conditions in which piping occurred. Afterwards, based on the analysis of the correlations between critical hydraulic gradient and the particle morphology or the grain size distribution, pore characteristics, the function relationship between the critical hydraulic gradient and the porosity (void ratio) of particular tailing silt was measured when the piping generates, and then a generalized computing equation of critical hydraulic gradient was exported. Finally, the reliability of the general formula calculation results is properly verified by laboratory test, and the results show that the measured value is basically in agreement with the calculated value. Therefore, this calculation formula can be used to estimate the critical hydraulic gradient of piping-typed tailing silt, and the estimation result is on the safe side.