The impact of windows to wall ratio and thermodynamic performance of windows on the insulating layer thickness cannot be neglected in existent residential building energy-saving renovation because of the bigger difference of windows to wall ratio. The limited equivalent coefficient of envelope heat transfer was posed based on “Design standard for energy efficiency of residential buildings in severe cold and cold zones”. Factors of orientation, windows to wall ratio and windows types should be comprehensively considered in building calculation model of thermal insulation thickness of external wall and envelope thermodynamic performance optimization model. These models are validated by an existing residential building in Xuzhou.