Using the monitoring data, an expression of ground settlement with associated coordinates above shield tunnel during construction was established, in which coefficients were obtained by the least square method. After transforming this expression into a standard form of elliptic umbilic catastrophe model, the instability criterion for the soil above shield tunnel was determined with the analysis of this model. Instability criterion values were thereafter obtained then the soil state would be judged according to these values. Three cases in different regions in China, referred to Bund Channel in Shanghai, metro line 1 in Chengdu City and metro line 4 in Beijing, were presented. They showed that the method developed in this paper was suitable in analyzing and forecasting the stabilities of soil above shield tunnels.