Crack width can directly represent the damage characteristics of structural components， and it is an important index for seismic damage assessment. In view of the many shortcomings in the calculation methods and measurement of crack width， considering a variety of influence factors， a calculation method of diagonal crack width based on the bond-slip theory is proposed. In view of the uncertain load and the unaccurate calculation of crack width of the member in service， the calculation method and flow chart of transverse crack and oblique crack width of reinforced concrete （RC） column based on bending deformation and shear deformation are presented. On this basis， considering the real state of the RC column under the axial-flexure-shear interaction in earthquake， the theoretical relationship between the displacement and the crack width is established， and the method and process of calculating the maximum crack width according to the total horizontal displacement is proposed. The experimental results show that the maximum crack width of the RC column calculated by the proposed method is accurate and can reveal the dynamic evolution process of the crack width with the increase of displacement under the state of axial-flexure-shear interaction. In addition， the failure mode of RC column can be judged according to the numerical ratio of transverse crack and oblique crack.