Abstract: The emergence and development of soil shear bands is one of the concerned problems of geotechnical engineering. Consolidation drained shear tests on phyllite detritus soils are conducted, and the real-time monitoring scan is performed in the test process by using the CT technology. A series of CT pictures at the same section position are obtained under different strain conditions. It can be observed that the shear band obviously appears in the specimen with the increase of strain from CT slices. Using the CT technology, the process from local crack to form the complete shear bands of specimen is reproduced under the axial force. Through the analysis of the average value of CT number and standard deviation of 7 regions in the CT pictures, it can be found that the range of axial strain of 3%~10% is the main stage for formation and development of shear band . The specimen is mainly compression when the strain is below 3%, but when the strain is over 3%, the specimen gradually shows local crack under the deviatoric stress, and the crack is more and more obvious with the increasing strain. The strain is not equal between the CT statistical values and macro cracks through macroscopic observation. It is suggested that the CT statistical values should be regarded as the criterion to determine the strain of micro cracks in the shear band.