Abstract: The Kelvin-Voigt model was taken as the rhelogical model of large deep tunnel support,and the displacement time-equations of tunnel support were derived.The time-reliability of the tunnel support was defined,i.e.,the ratio of the length of the arc AB and OB.The length of AB was the distance between the intersection A,which was the displacement time–curve(U r(t i)) and the special value of displacement,and the displacement converging valve.The length of OB referred to the distance between the action position O of surrounding rock and the support and the converging valve of displacement.On such a basis,some real projects were calculated using computer programs.The results show that the failure probability of the tunnel support is very sensitive to the elastic modulus of surrounding rock and the tunnel support,and it is also sensitive to the supporting height.The failure probability of the tunnel support with the same supporting strength will reduce when the elasticity of surrounding rocks reduces.