Abstract: The structural parameter was applied to engineering firstly based on its analysis and modifiability.The variation of structural characteristics for excavated tunnels was studied.It was shown that the structural parameter was strongly affected by the diameter and depth of tunnels.The structural parameter in vault direction decreased firstly and then increased along with the tunnel depth.There was a minimum value of this paraneter.Compared with the structural parameter before excavating,its loss came to the maximum at the vault top of 1.7～2.5 m.The disturbance of structural characteristics for tunnels with larger diameter was more serious than that for small one.Although the minimum value of structural parameter did not obviously change,and its position moved gradually apart from the vault.Finally a new method to estimate broken zone according to the structural parameter was proposed.It was found that the thickness of broken zone of loess tunnels was about 1.7～2.5 m.