Abstract:The recent progresses in non-equilibrium statistical mechanics are summarized and an attempt is made to build a new descriptive method based on a new variational principle. By employing Maximum Flux Principle, as modified Hamilton principle for open systems, a new statistical ensemble is developed and a new potential function is defined. Then the Langevin equation and the renormalization equation are obtained, through which the fractal structure is ultimately derived. A new framework of statistical mechanics is therefore outlined. With an example of quantitative analysis of urban system, it shows advantages and potentialities in dealing with universal open complex systems.