Abstract: In order to keep the safety construction TBM tunneling and avoid surface settlement and failure accidents at the TBM Portal areas,This paper carries out studies on how to determine the length of longitudinal reinforcement scopes and establish a new stability model for sandy stratum for the portal areas.It analyzes and summarizes the limitations of existed theoretical models.It refines some existing conclusions.It further presents a new load equivalent and sliding failure models based on strength and stability theories.The new theoretical models can better reflect the mechanical characteristics and deformation modes of actual cases and have a good stratum adaptability.Meanwhile,in order to certify its rationality of improved strength model,the two strength models have been put into use for calculating one actual engineering stratum reinforcement of TBM portal areas.Sensitivity analysis on the main influence factors of existed and new improved models have also been made.The conclusions are: when the diameter of shield tunnel is small,the deviation of calculation results of existed and the new models are small.But as the diameter of shield tunnel is continuously increasing,the deviation of existed model is bigger,which may cause serious accidents if the existing models are applied into actual engineering.So,for large diameter shield tunnel engineering,especially when the diameter is lager than 10meters,the new improved model will be better for designing and quantifying the longitudinal reinforcement scopes.It has the big significant for engineering construction.The load equivalent model presented in this paper gives a new analyzing and computing method for studying load asymmetric problem based on strength theory.It also provides a good reference for similar problems.