Abstract: Rubber cylinder is the key part of the grout injector with mechanical sealing. Its deformation process in borehole has multiple nonlinear characteristics. Analysis by the finite element software showed that along with the increase of axial displacement in borehole, the rubber cylinder was firstly contacted with hole wall, then the inner tube, finally extrusion deformation may occur. For sealing the borehole effectively and ensuring the resilience of the rubber cylinder, the compression displacement should be controlled between 17.8mm~19.4mm. Application shows that it achieved a good blocking effect by controlling the compression of the rubber cylinder under this range, and effectively extended the life of the rubber to improve the reliability of grouting.