An on-the-fly trajectory surface hopping dynamic simulation has been performed for revealing the multi-state nonadiabatic deactivation mechanism of coumarin. The mechanism involves three adiabatic excited states, S3(ππ∗Lb), S2(nπ∗, ππ∗La) and S1(ππ∗La, nπ∗), and the ground state S0 at the four state-averaged complete active space self-consistent field, SA4-CASSCF(12,10)/6-31G∗ level of theory. Upon photoexcitation to the third excited state S3(ππ∗Lb) in the Franck-Condon region, 80% sampling trajectories decay to the dark S2(nπ∗) state within an average of 5 fs via the conical intersection S3(ππ∗Lb)/S2(nπ∗), while 20% decay to the S2(ππ∗La) state within an average of 11 fs via the conical intersection S3(ππ∗Lb)/S2(ππ∗La). Then, sampling trajectories via S2(nπ∗)/S1(ππ∗La) continue with ultrafast decay processes to give a final distribution of quantum yields as follows: 42% stay on the dark S1(nπ∗) state, 43.3% go back to the ground S0 state, 12% undergo a ring-opening reaction to the Z-form S0(Z) state, and 2.7% go to the E-form S0(E) state. The lifetimes of the excited states are estimated as follows: the S3 state is about 12 fs on average, the S2 state is about 80 fs, and the S1 state has a fast component of about 160 fs and a slow component of 15 ps. The simulated ultrafast radiationless deactivation pathways of photoexcited coumarin immediately interpret the experimentally observed weak fluorescence emission.