The particle flow discrete element models for uniaxial compression and tensile tests of rocks are established to study the influence of the particle distribution randomness on the macroscopic mechanical properties of such model. The results of macroscopic mechanical properties show strong discreteness due to the variance of particle distributions, in which compressive strength, tensile strength and Poisson's ratio follow the normal distribution and the Young's modulus follows the negative skewness distribution. The average values of the macroscopic strength obtained based on multiple calculations with different particle distributions should be used for the calibration of microscopic parameters. According to the relationship between sample size and deviation of macro strength averages, the minimum calculation number required to obtain high-precision macro strength with different confidence levels is given.