Abstract: The mechanism of triggered microseism by excavation of deep rock mass is discussed from the view of energy release. Results reveal that the elastic strain energy release of rocks, accompanied with the formation of new free surface during the rock fragmentation by blasting, is a transient process; and in highly stressed rock mass, the triggered microseism by blasting excavation is attributed to the coupling of blasting load and transient release of in-situ stress (TRIS). The TRIS-triggered microseism could become absolutely the main component of total vibration, which depends on the storage capacity of rock energy, the excavation method and the size of excavated surface. In combination with the blasting excavation of an underground powerhouse in the Pubugou Hydropower Station, the microseism excited by TRIS is identified through time-energy density analysis and amplitude spectrum analysis of monitored microseismic signals in surrounding rocks. Results indicate that lower frequency component in the coupled microseism results more from TRIS than from blasting load, and higher frequency component originates from the blasting load alone. The coupled microseismic signals are separated by employing the finite impulse response (FIR) filter, and separated waves agree very well with numerical simulation results.