Looking for new sources, especially coherent light source with short-wavelength has aroused close attention. From the views of potential and field , the motion equations of particles is reduced into Mathieu equation in the framework of classical mechanics and linear approximation. The periodic solution and corresponding transition curve are calculated by using method of multiple scales, the stability of the system is discussed. The results show that the system stability related to curvature of bend crystal, the curvature the greater, stability of the system worse. This means that system stability must be fully considered when people tries to use a crystalline undulator radiation as coherent light source with short-wavelength.