Blasting is a major means for excavation of rock slope, and the blast-induced damage to reserved rock mass must be strictly limited to ensure safety of the high slope and reduce the cost for support. As a traditional and widely used technique, the sonic wave testing is usually adopted to detect the extents of blasting damage, but the workload of detecting is considerably heavy during excavation of large-scale rock slopes with a height of several hundred meters. Thus, a simple but efficient method of blasting damage assessment based on comprehensive vibration survey was presented in this paper. In total, 5 bench blasting experiments were conducted at the excavation site of the left dam-abutment slope of the Bai-he-tan Hydropower Station in southwestern China. A semi-empirical and semi-theoretical approach for predicting the blasting damage depth based on the Peak Particle Velocity (PPV) of blasting vibration at certain distance was established. The method was used to predict the damage depth of the subsequent bench blasting with monitored vibration, and the predicted results agreed well with those obtained by field damage testing, which indicated that the method proposed in this paper is reasonable and credible. Although there is no rigorous theoretical basis, this real-time damage assessing approach is simple and convenient to use, and it can significantly reduce the massive workload of sonic wave testing and greatly improve efficiency. The accuracy of damage assessment is heavily dependent on the engineering geological conditions of excavation site and the vibration monitoring quality. Thus, careful investigation of the engineering geological conditions before blasting excavation and field experiments are necessary for the application of this method.