To the tunnel with pressure in plastic rock body, it is very important to determine accurately the thickness of the smallest overlay stratum by controlling the radius of plastic zone, according to the intensity and the value of inner pressure of rock body. On the basis of the comment on classical geager method and the description of Hoek- Brown strength criterion to the destroy of rock body and the assumption supposed by Hoek in 1980, the calculation formulas of the stress in plastic area, the radius of plastic zone and the smallest overlay stratum thickness are deduced with the elastic - plastic theory and the application method is explained with an simple example.