Abstract: By visual reformation on traditional shear instrument, and combining digital image deformation measurement technology, it studies the forming mechanism and expansion processes of shear bands of sand samples with different sizes in shearing tests. The effects of sample sizes on sand strength are concluded. With the typical sample size, the evolution laws of shear bands (especially for secondary shear bands) in different shearing courses and various normal stresses are summarized. And the force analyses of arch damage areas discovered in tests are carried out. The strength parameters of primary and secondary shear bands are compared. The results show that sample sizes have influences on strength parameters. When the ratios of long to short side are greater than 2, test results deviate from average. When the ratios of long to short side are greater than 4, serious errors are caused. The shear failure areas are not always developed along horizontal shearing gap. The upward arch secondary shear bands are gradually formed in middle and lower part of upper boxes, and shear failure areas are developed along the primary and secondary shear bands. The arch failure areas decrease with the increasing shear displacements and decreasing normal stresses. The normal and tangential stresses on secondary shear bands are larger than those on primary shear bands. The strength indexes of primary and secondary shear bands are basically the same, which indirectly proves the rationality of the existence of secondary shear bands.