The photodissociation dynamics of cyclopropanone was explored with the complete active space self-consistent field (CASSCF) calculations and ab initio nonadiabatic molecular dynamics simulations. The related minima, transition state (TS) and minimum-energy conical intersections (MECIs) were obtained as well as energetics. In the static CASSCF calculations, one MECI was found to be responsible for the fluorescence disappearance below 365 nm because the ultrafast internal conversion (IC) via this MECI deprived the opportunity of the fluorescence emission. Further evidence of this ultrafast IC event came from the subsequent ab initio nonadiabatic molecular dynamics simulations.