Xikema Tunnel,with a length of 2812 m,is the longest tunnel along of high Shanghai-Beijing speed railway.The surrounding rock of the tunnel is mainly Cambrian Limestone.Targeting at a real pragmatic problem that sometimes the rock mass integrity index is more than 1 and taking Xikema Tunnel as an example,the rock sample wave velocity-stress relation tests are carried out.Then unloading index is put forward and formula of raock integrity index is modified so as to improve rock quality classification method.In the end,the improved rock quality classification method is applied to Xikema Tunnel.The result shows that the quality of most rock masses here is level Ⅲ or Ⅳ.