The shear strength parameters of rock discontinuities have variability is a common sense and the shear strength variability is difficult to obtained due to the cost of direct shear tests. It is one of the main reasons restricting the development of rock slope reliability evaluation. In view of this, combined with a small number of measured rock discontinuities roughness curve samples, rock mechanical parameters, the JRC-JCS (joint roughness coefficient-joint compressive strength) shear strength model, random field theory and non-invasive discrete element numerical experiments, a method to obtain the variability index of rock discontinuities shear strength parameters was proposed. Applying this method to the reliability evaluation of a rock slope at the entrance section of a tunnel, The result show these as follows. The variance of the ordinate roughness of the main control rock discontinuities of the rock slope is 0.012 5 cm², the correlation distance is 0.75 cm, and the generated random roughness curve is similar to the measured sample. The average cohesion and internal friction angle of the rock discontinuities are 0.427 MPa and 54.9°, the standard deviations are 0.229 MPa and 4.47°, and the coefficients of variation are 53.63% and 8.14%, respectively. The slope stability coefficient obeys the normal distribution, the instability probability is 2.8%, and the reliability is 1.52.