In this article, structures and energies of cyclopropenone in the low-lying electronic states have been determined by the CASSCF and MS-CASPT2 calculations with different basis sets. Two minimum-energy conical intersections (CI-1 and CI-2) between S0 and S1 were obtained and their topographic characters were characterized by the SA4-CAS(10,9) calculated energy gradients and nonadiabatic coupling vectors. The AIMS method was used to carry out nonadiabatic dynamics simulation with ab initio calculation performed at the SA4-CAS(10,9) level. On the basis of time evolution of wave functions simulated here, the S1 lifetime is fitted to be 125 fs with a pure exponential decay for the S1 electronic population. The CI-1 intersection is mainly responsible for ultrafast S1→S0 nonadiabatic transition and the photoinduced decarbonylation is a sequential process, where the first C-C bond is broken in the S1 state and fission of the second C-C bond occurs in the S0 state as a result of the S1→S0 internal conversion via the CI-1 region. As a minor channel through the CI-2 region, the decarbonylation proceeds in an asynchronous concerted way. Effects of the S1 excess energies and the S1-S0 energy gap on the nonadiabatic dynamics were examined, which reveals that the S1→S0 nonadiabatic transition occurs within a small energy gap and high-energy conical intersection regions can play an important role. The present study provides new insights into mechanistic photochemistry of cyclopropenones and reveals that the AIMS dynamics simulation at a high-accuracy ab initio level is a powerful tool for exploring a mechanism of an ultrafast photochemical reaction.