The dynamical properties of light phase in the system of a cascade three-level atom interacting with a single-mode coherent state field are studied. The influence of the atomic motion,the field-mode structure,and the intensity of the atom-field coupling on the phase probability distribution and the phase diffusion are discussed. The results show that the dynamical properties of light phase are determined not only by the intensity of the atomfield coupling,also by the velocity of the atomic motion and the parameter of the field-mode structure through the time factor g■(t)=2^－1gt－(4εp)^－1sin(2εpgt).