Abstract: The Lagrange’s equation is widely used in solving complex dynamic problems. As soon as the kinetic energy of a system at a general position is known, the governing equation of the system can be easily obtained by the Lagrange’s equation. But to calculate the kinetic energy of a system at a general position is not easy in many cases, which is an obstacle to the application of the Lagrange’s equation. From a theoretical analysis, we find that to the dynamic problems of the velocity-free release, the Lagrange’s equation provides a simple way of solution.