The waterrich Xigeda formation in PanzhihuaXichang Region is characterized by soft and easysliding and the difficult tunnel portal excavation due to slope cracking. Hence, the schemes of anchor pile, jet grouting pile and PBA (piles, beams and arches) method are tentatively used for shallowburied waterrich Xigeda formation of Qianjiashan Tunnel on ChengduKunming Railway. The surface cracks, largescale deformation and creep deformation of steep facade excavation occur when adopting anchor pile method and jet grouting pile method, while the instability of waterrich Xigeda formation can be guaranteed by adopting PBA method for opencut tunnel and cavity backfilling. The applicable and analytical results show that: (1) The difficult tunnel portal excavation in Xigeda formation are mainly induced by tunnel crosssection instability caused by bilateral empty space and tunnel longitudinal instability caused by unilateral empty space. (2) The PBA method has clear mechanical mechanism, which can guarantee instability of tunnel portal excavation effectively.