Besides a channeling radiation there is an undulator radiation in an periodic bent crystal. A sine-squared potential is introduced to describe the motion behaviours of the particle in the system,the particle motion equation is reduced to the pendulum equation with a dampping and a force terms in the classical mechanics frame and the dipole approximation. The bifurcation and the chaotic behaviours of a separatrix orbit with the Smale horseshoe are analysed by Melnikov method. The critical condition of system through to a chaos is found. It shown that the critical condition of the system entered in a chaos is related to its parameters,provided regulating a parameters of the system,the chaos can be avoided or controlled in principle. Thus the effective export of X-laser or γ-laser can be assured furtherlly.