Vertical drilling technology is commonly used in mountain tunnel investigation with complex geological structure， however， it is difficult to meet the requirements of the terrain change observation adits investigation. Taking an terrain change observation adit investigation project in Xichang as an example， a method to solve the two key problems of detailed surrounding rock classification and water inflow estimation in tunel investigation based on horizontal drilling technology is put forward. Detailed logging of the joint fissures in full-hole horizontal drilling cores and the joint fracture linear density statistics are collected. Combined with the developed orientation and space of the joint fractures in the project site area， the standard rock mass model is established. Through statistical analysis of the standard model， the conversion factor K of the one-dimensional core fracture linear density and the three-dimensional volume joint number J_v of the rock mass is finally obtained. After that， the rock mass integrity index K_v and BQ values of the tunnel surrounding rock are calculated according to the standards and specifications， and then the classification of tunnel surrounding rock can be more detailed divided. In the process of drilling， horizontal drilling can be used to accurately judge the location of tunnel water inflow and measure it， which provides accurate calculation parameters for the calculation of tunnel water inflow.