To investigate the adaptability of an open tunnel boring machine(TBM) excavation diameter to unfavorable geological conditions in plateau areas, the reasons for limit invasion caused by primary support deformation in the TBM tunnel are analyzed using existing problems and construction data from the  completed and underconstruction projects. The abovementioned data and lining parameters are compared with those in the case study of a plateau railway tunnel. It is discovered that TBM shield shrinkage and the support capacity formed using primary support system are the key causes of limit invasion for primary support under unfavorable geological conditions. Furthermore, when the TBM excavation diameter reaches 10.2 m, the sections with mediumandabove rockburst in grade Ⅲ surrounding rock, the joint intensive zone of grade Ⅳ surrounding rock, and the altered rock area of grade Ⅴ surrounding rock are expected to be prone to limit invasion. Therefore, the following suggestions are made: (1) Shield shrinkage should be considered when designing TBM excavation diameter. (2) Steel segment can provide temporary support for primary support in strong rockburst sections, dense joint zone, and altered rock and shorten the formation time of support capacity; The TBM excavation diameter should be increased to provide space for steel segment installation.