The mechanism analysis of segment upfloating during construction is conducted. The main cause of segment upfloating in softandweak stratum is formation rebound induced by stratum stress redistribution; the variation rule of tunnel upfloating amount and the influence of stratum characteristics on segment upfloatation are analyzed by dynamic model of tunnel excavation; the elasticity modulus and the amount of segment upfloating present exponential correlation; the influence of elastic modulus on segment upfloating is far greater than that of the formation cohesion strength and internal friction angle; the formation resilience force caused by shield tunneling, the amount of segment upfloating and the affected height of tunnel upfloating are calculated by force balance theory; finally, the feasibilities of the formulas are verified by case study of Shiziyang Tunnel. The results can provide a new theory and method for predicting the shield tunnel segment upfloating caused by formation rebound.