Abstract: To study the coal mine goaf and evaluate the suitability about goal surface used as a building sites, the article proposes the concept of goaf system and establishes its model. The system's stability is analyzed theoretically from three aspects of time, space and external environment, and the effect to the system is discussed quantitatively by coal pillar, overlying rock, floor, ground deformation and upper loads. For easy operating, the evaluation of coal surface is classified into two levels, named "usability or not" and "reinforcement or not", and the line of work and the standard of evaluation are provided as well.