Since the ancient times, schools of thought stands like a forest, prosperity and air views together, and different ways lead to the same goal, such as Newton and nonNewton schools, and differential and nondifferential schools in dynamics. In this paper, the concept of kinetic differential was proposed, and the generalized principle of kinetic energy(GPT)was obtained, which is compatible with the virtual velocity in Lagrange′s original work (Analytical Mechanics). Its applications in aerospace dynamics and ground mechanics were expounded, which indicated that the GPT is an actual, easy and general principle of dynamics in the multidegree of freedoms (MDF) systems with variable parameters.