The problem of large deformation in high-stress soft rock tunnels has become the key to constraining the project's progress and increasing the cost. Based on a plateau high-stress carbonaceous slate soft rock tunnel, the mechanism of large deformation, rock mechanical parameters, and large deformation treatment technology were studied in this paper. The results show that: (1) The large deformation of soft rock tunnel peripheral rock is mainly related to the modulus of elasticity, Poisson's ratio, excavation radius, and the level of ground stress; the radius of the plastic zone in addition to the above factors is also related to the cohesion of the rock body, the angle of internal friction, and the resistance of support. (2) Through indoor uniaxial and triaxial tests, the average value of elastic modulus of the rock body of the dependent project was determined to be 9.003 GPa, Poisson's ratio to be 0.246, the angle of internal friction to be 36.09°, and the cohesion to be 4.79 MPa and the above values were taken as the input parameter values for numerical calculations. (3) According to the engineering analogy, the design change treatment method of “early closure of the initial support, prestressed long anchor cable + double-layer initial support release resistance combination of active and passive control” is given for the large tunnel deformation of the dependent project. (4) According to the original design, the maximum cumulative horizontal convergence value reached 72 cm, and the cumulative amount of vault subsidence reached 38 cm. The test data of the test section after adopting the change of treatment method showed that the deformation was controlled at the level of convergence of 11 cm, and the value of vault subsidence was only 3.5 cm, which achieved the purpose of controlling the large deformation and making the tunnel progress steadily. The research results can provide a reference for the control of large deformation in tunnels with similar lithology.