The macro- and meso-mechanical behaviors of shear band were simulated using discrete element method. Loop structures were taken as the basic meso-mechanical unit of granular material after tessellation. The evolution of numbers，geometrical morphology，mechanical characteristic and sliding stabilities for different loops during shearing were analyzed in details，and the essential relationships were discussed between the variation of loops and the strength，dilatancy of granular material. Numerical results show that the variation of numbers and geometrical morphology of loop structures are approximately consistent inside and outside the shear band during shearing，but the degrees are different，the mechanical states and sliding stabilities of different loops inside and outside the shear band are obviously different. Proportions of higher-order loops are increasing and shapes of them are more flat，while proportions of lower-order loops are decreasing and shapes are keeping unchanged. The macro-scale dilatancy is reflected by the conversion from lower-order loops to higher-order loops in meso-scale mechanical structure，which forms a kind of self-organized structure under specific force and boundary condition. Two kinds of definition of sliding rate were proposed to analyze the sliding stabilities of loop structures. Evolution of sliding rate inside the shear band after peak stress state is obviously different from that outside the shear band，and the location of maximum sliding rate are not coincide with the location of peak stress point on stress-strain curve. The strong force chains，which are formed by higher-order loops，undertake larger load and have higher sliding stability than lower-order loops. The contact sliding and recombination during the conversion from lower-order loops to high-order loops are the main causes of plastic deformation for granular material.