Abstract: The transverse deformation of shield tunnels may ultimately threaten the safety of tunnel structure. The surcharge above the tunnels is one of the leading factor to cause their transverse deformation. The evolution of transverse defromation of the tunnels under surchage is firstly studied using numerical simulation considering the effect of the elastic resistance of surrounding soil and lateral coefficient of earth pressure at rest. It is found that the development of joint opening and the stress states of both tunnel concrete and bolts are dependent on the variation of tunnel diameter. Thus, the variation of tunnel diameter can be taken as the assessment index to predict the performance of shield tunnels in soft soils. The criteria for tunnel assessment are determined in terms of diameter change. Then, a geometry-based simple method is proposed to study the relationship between joint opening and tunnel squating based on the tunnel crown settlement. The method is verified by the above- mentioned numerical simulation. This simple method provides a very convenient way to estimate the safety state of shield tunnels.