Abstract：At present it is universally considered that occurrence of self-organized criticality (SOC) relates to scale effect and organization principle. Scale effect bases on the sand-pile experiments performed around the world；but the organization principle hasn't been discussed ever. First，by means of the sand-pile experiment on a slope，it is discovered that self-organized criticality in granular mixtures is strongly influenced by the nonuniform degree of granular material. With the gradual increase of the nonuniform degree，the experiment presents the different phenomena and statistical results. Meanwhile，the dynamical behavior of granular material can transform from quasi-period to SOC and then to quasi-period again. Second，a cellular automata (CA) model that can describe the nonuniform property and embody the nonuniform coefficient is designed；i.e.，the value induced by friction is introduced as a token of different roughness of the sand pile surface. Adjusting the value of energy dissipation and the radius，experiments with different nonuniform coefficients and different scales can be simulated. By comparison the simulation results are similar to those obtained from the sand-pile experiment；it is found that they are similar and the organization principle is concerned with the robustness and the sensitivity of system. Only when the nonuniform granular mixture maintains the robustness and the sensitivity of whole system，it can break through the limit of scale；and the large-scale system remains self-organized criticality. Once the nonuniform coefficient continues to increase，the robustness dominates in the system and the sensitivity vanishes or weakens，the balance is broken，the nonuniform granular material can't show SOC. Finally，it can be achieved that the balance of robustness and the sensitivity is one of necessary conditions that make system show SOC.
