Abstract: Typical large deformation geological hazards of soft rock occurred frequently during tunneling in Yangjiaping Tunnel,including extrusion at the tunnel wall and severe damage of the primary and secondary support. This paper uses the combined method of the engineering geological investigation with field monitoring and measurement,laboratory rock mechanics test,numerical simulation and microscopic analysis. It reveals the formation mechanism of large deformation of Yangjiaping Tunnel. The field monitoring results show that there might be a close relationship between the large deformation phenomena and tunnel seepage. Furthermore,the laboratory test results show that the longitudinal wave velocity of phyllite are attenuated by 2.04% ~5.85% after immersion. After being fully immersed in water for 28 days,the uniaxial compressive strength and elastic modulus of the phyllite decreased by 59.56% and 69.68%. The water-rock interaction results in certain damage of the phyllite structure. Meanwhile,we use the X-Ray Diffraction to detect the mineral composition,and to reveal the microscopic reasons for the reduction of the surrounding rock strength after water-rock interaction. The results from in situ stress tests and inversion analysis indicate that as a result of high tectonic stress and low rock strength,92.08% of Yangjiaping Tunnel is in a state of high to extremely high geostress. In addition,the engineering mechanical properties of the high-steep thin layered phyllite stratum and water-rock interaction are the main reasons for the large deformations in the Yangjiaping Tunnel.