In order to solve the problems of tunnel instability caused by poor geological weak surrounding rock, the uncertainty of grouting parameters of weak surrounding rock, and the influence of grouting thickness of surrounding rock of ice water accumulation body of Milin tunnel on the safety of support force, the tunnel surrounding rock model with different grouting thickness is established  based on the threedimensional finite element FLAC3D simulation software. The surrounding rock characteristic curve after grouting is obtained, and the allowable support force corresponding to different grouting thickness is determined through the allowable deformation. On this basis, the influence of different grouting thickness of surrounding rock on the primary support structure is further discussed, and the safety factor of primary support at each position of the section is calculated. The results show that: （1） The greater the thickness of the grouting circle, the smaller the allowable support force based on the allowable deformation; and the worse the surrounding rock conditions, the greater the allowable support force corresponding to the same grouting thickness. It is suggested that the reasonable thickness of the grouting circle for the Ⅳ1, Ⅴ1 and Ⅵ surrounding rocks of the ice water accumulation section of the Milin tunnel should be 3.5 m, 3.5 m, and 5.0 m, respectively; （2） With the increase of the thickness of grouting circle, the safety factor of primary support corresponding to each position of surrounding rock at all levels increases. （3）The safety factor of primary support is generally large at the arch spandrel, crown, and bottom, and minimum at the arch waist and foot.