Kink band of metamorphic rock is an important geologic tectonic trace resulted from the crystal slip under the constructional stress fields. To study its formation mechanism and construction parameters is very important for exploration of tectonic movements of the earth crust. Due to the larger deformation in kinking band，the generally linear theroies are not availiable so the constitutive function of crystal plasticity in large deformaion were deduced;combined with the bifurcation theory，a continuum model of analysis on kink bands’ formation of metamorphic rocks under planar strain was established. An algebra equation for calculating angles of the kink band was deduced，morever it is solved. The results of the theoretic calculation are accordant with experimental results.