The invention provides a method for testing the frictional coefficient of rocks. The method comprises the following steps: I. manufacturing a cylindrical rock test piece of which the ratio of height to diameter is more than 2:1; II. putting the rock test piece into a vaccumized saturation device and making the rock test piece be saturated with a saturation liquid; III. mounting an axial deformation sensor and a radial deformation sensor on the rock test piece and putting the rock test piece into a triaxial chamber of a triaxial rock testing machine; IV: carrying out triaxial test to the rock test piece by the triaxial rock testing machine; V: switching off the triaxial rock testing machine, taking out the fractured rock test piece and measuring the included angles of the fracture surface and the end surface of the test piece; and VI: calculating the frictional coefficient of the rock according to the equation. The method can simulate the underground conditions to measure the frictional coefficient of the rock under a stratigraphic condition.