The asymptotic stability of nonlinear time-varying discrete-time systems was studied by using Lyapunov approach. If there exists time independent Lyapunov function V that is positively definite, and its difference is non-increased along the solutions of the systems, plus the additional condition imposed as the one of statements of the Barbashin-Krasovskii theorem, theconclusion of asymptotic stability will be obtained. In applications of Theorem 1 to the time-invariant and periodic systems respectively, the LaSalle theorem and Barbashin-Krasovskii theorem can be reobtained.