Abstract: Abstruct Determined the probability distribution of coal floor rock mass mechanical parameters correctly is the premise of random uncertainty analysis of coal mining on the confined water barrier ability of bottom water.Based on the test results of lower group coal water-resisting floor rock mass(mudstone, siltstone and sandstone as representatives) mechanical parameters, the normal distribution, lognormal distribution and weibull distribution were used to fit the probability distribution, and k-s test was used to estimate the goodness of fit. On this basis, based on the thickness weight of three kinds of rocks in the water-resisting floor, the mean and variance of floor rock mechanics parameters were deduced. The results showed that: the best fit of water-resisting floor rock strength and deformation parameters is normal distribution, followed by lognormal and weibull distribution. The Poisson's ratio and elastic modulus of the overall floor are 0.35 and 36.46GPa, the variances are 0.01 and 1.15 respectively; the mean and variance of overall compressive strength are 37.82MPa and 583.41, of tensile strength are 2.62MPa and 2.23, of cohesion are 2.13MPa and 0.96, of friction angle are 32.93and 1.96.