Abstract: Based on the planar characteristics of digital image processing technique,using fly ash as test specimen,the shape of triaxial specimen,the stress-strain characteristic and formation conditions of shear band were studied.In order to make analysis and comparison,triaxial specimens with different densities,cell pressure and degree of saturation were chosen and 20 groups of consolidated drained triaxial tests were performed on improved high accuracy triaxial instrument.It was shown that there were various shapes of shear band and no certain rule could be followed.During the test,the stress-strain of the existing shear band exhibited obvious peak value,the shear band appeared before the peak value,and the strength tended to be residual strength after the formation of shear band.The necessary condition under which shear band might appear in fly ash specimen was the relatively large density,small cell pressure and relatively high degree of saturation specimen.The digital image processing technique could help to record the whole process of the formation of shear band of triaxial specimen.Then the time of appearence and complete formation of shear band could be determined easily.