The invention discloses a method for judging the geological layer where the drill of a drilling machine drilling for a certain depth is positioned. The method comprises the following steps: 1), obtaining the rock formation density of a target rock formation, P wave velocity and Poisson's ratio Mu, so as to obtain the elastic modulus of the target rock formation through calculation; 2), comparing the elastic modulus of the target rock formation with data in a computer database, so as to obtain the type of the rock formation through analytical judgment; 3), recording the depth when the elastic modulus of the rock formation and the original value of the elastic modulus of the rock formation change suddenly, wherein the depth is the interface depth of the rock formation; 4), repeating the steps (1, 2 and 3), so as to obtain the geological layers where the drill of the drilling machine drilling for different depths are positioned. The method has the advantages that physical properties and mechanical parameters can be obtained through laboratory tests without depending on rock core, real-time analysis and calculation can be carried out according to corresponding data collection when the drilling machine is used for drilling, and the rock formation where the drill is can be judged, so that the process of obtaining the physical properties and mechanic parameters of rock formations of different depths can be simplified greatly, and engineering time is shortened.