An external periodic field has been introduced in the Seeger equation; a motion equation of a crystal dislocation has been induced to Duffing function with the hard—spring properties.A global bifurcation of the system and a probable way into the chaos have been discussed by using Melnikov methoud. It shows that the system enters into the critical state , then enters the chaos by the cascading bifurcation, if δ/α is defined and Ω is reduced continuously.