Abstract: It is one of the key problems to identify strength parameters C and  of structural planes controlling the stability of rock block in rock slope stability analysis. At present, the parameters are calculated with the linear weight method by linking length and non-linking length. It can cause an error about 40% with the actual strength values. This paper uses 11 sets of the linking ratio: 10%, 30%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% and 90%. Blocky samples with dimension of 20 cm10 cm5 cm are made using M15 mortar. Further, shear strength testing is carried out in laboratory. The non-linear testing curves between C and  are obtained for eac of the control structural plane and the linking ratios. According to testing results, a method to calculate C and  for the control structural plane is established, where the linking ratio and the safety grade of control engineering are also considered. It is named as the linking ratio method. In use of the method, the linking ratio of the control structural plane is to be measureed in-situ. The axial compressive strength and frictional angle of the intact rock can be obtained from conventional laboratory rock test. The linking ratio method can be used with ease. Calculated results are compared well with actual strength values.