In this study, the slope piping experiment selected 6 different sands in mining engineering of civil construction. In addition to discussing the physical parameters of the graded sands in mining engineering, the piping destruction parameters of different slopes were obtained from the experiment. When piping occurs, we can use numerical model to get the hydraulic gradient value at the failure point. The difference between the theoretical estimation and the numerical model was compared and we found that different particle size distribution caused the main difference. The particle size parameter ratio, D*, is used for correcting the critical hydraulic gradient value in this study. We learned the correction coefficient of D* is between 0.5~0.857 in different tests and its value approximates the reciprocal of the coefficient of uniformity. Therefore, the critical hydraulic gradient estimation formula for the slope piping of the uniform sand can be future corrected, and it can be used for estimating the graded sand in this study. In civil engineering the sand filling project can use the estimation formula of this study to calculate the critical hydraulic gradient as the reference for design and disaster prevention.