GIT (groundwater inflow into tunnels) is one of the important factors that have adverse effects on the construction and operation of tunnels, which can result in mechanical, physical and chemical effects and has impact on the hydromechanics and deformation characteristics of the surrounding rock masses. GIT can result in strength reduction, expansion and deformation of the surrounding rock mass, even instability and failure of the surrounding rock masses. Taking superlong tunnels with great covers as example, the authors make systematic analysis on the effects of GIT on the characteristics of the surrounding rock masses, including strength, deformation and expansion characteristics of the surrounding rock masses, on basis of analysis on the characteristics of water inflows and laboratory experiments and field measurements. The conclusion indicates that the larger of the water inflow rates and the worse of the rock masses, the greater the effects on the characteristics of the surrounding rock masses are.