When slopes are excavated in rock masses with the blasting method, blasting and stress relaxation will produce an excavation damage zone (EDZ) in the remaining rock mass. This study presents an estimation of the rock mass properties in the EDZ based on the generalized Hoek-Brown failure criterion and the varying disturbance factor. In this estimation, the disturbance factor is quantified by using the acoustic P-wave velocities tested in undamaged and damaged rock masses, instead of a single constant value chosen from the descriptive guidelines. Statistical analysis of the acoustic test data shows that with an increase in depth behind the slope face, the disturbance factor declines from 1.0 to 0 almost in a linear manner. For blasting excavation of slopes in weaker rock masses, the zone in the immediate vicinity of the slope face is fully damaged and thus over this range the disturbance factor remains the constant value of 1.0. As the disturbance factor linearly decreases within the EDZ, the deformation modulus, uniaxial compressive strength and equivalent cohesive strength of the damaged rock mass linearly increase to the undamaged values. By assigning the varying parameters to the rock mass in the EDZ, a numerical calculation based on the FLAC program is conducted to assess the slope stability. It is found that using the varying rock mass parameters within the EDZ will yield completely different failure surfaces and factors of safety from that uses the constant parameters. Therefore, it is significant to develop such an approach to quickly estimate the varying rock mass properties in the EDZ particularly when in-situ tests are not available.