The process of taphrogenic differential settlement can be regarded as limited system. The authors in this paper,based on dynamic characteristics of taphrogenic differential settlement in geological skilled works' school, Xi'an city, by means of two-dimension limited system theory, processes monitor data and forecasts life circle or life span of settlement.The paper analyzes the affect of taphrogenic differential settlement 45 years later, which come from forecast, on subway tunnel, simulates it which limited system theory incomputer, and make a conclusion of that taphrogenic differential settlement will break subway tunnel 45 years later. Therefore, we must adopt effective measure to limit exploitation of deep pressing groundwater, and strengthen the construction technology and materials of subway tunnel. Thus we can ensure subway move smoothly tomorrow. The paper furnishes the reference material for building subway and it's running.