The stability of space charge and beam is one of basic problems for strong beam technology. The problem can be minimized by using sheet beam focus and transport in the periodic field. By introducting non linear and motion damping, the motion equations of the sheet beam in the cold cavity was reduced into generalized Mathieu equation in the framework of classical physics. The Smale horseshoe chaos and subharmonic bifurcations were analyzed by Melnikov method; and the critical conditions of the system were discussed. The results show that the stability of the system is related to its parameters, so only adjustmenting parameters can guarantee the system stability.