Based on the analysis and description of vertically-layered property of the fault, the conception of "structure depth" has been proposed for the first time, which may be used to describe the position in vertically-layered section for certain place of the fault. It is pointed out that the "structure depth" is also an important factor to determine the characteristics of the fault and its hydrogeological conditions, as well as the property, era, rock fades and scale of the fault.