Photoinduced chemical processes upon FranckCondon (FC) excitation in tetrakis(trimethylsilyl)-cyclobutadiene (TMS-CBD) have been investigated through the exploration of potential energy surface crossings among several low-lying excited states using the complete active space self-consistent field (CASSCF) method. Vertical excitation energies are also computed with the equation-of-motion coupled-cluster model with single and double excitations (EOM-CCSD) as well as the multireference MollerPlesset (MRMP) methods. Upon finding an excellent coincidence between the computational results and experimental observations, it is suggested that the FranckCondon excited state does not correspond to the first pi pi* single excitation state (S-1, 1(1)B(1) state in terms of D-2 symmetry), but to the second B-1(1) state (S-3), which is characterized as a sigma pi* single excitation state. Starting from the FranckCondon region, a series of conical intersections (CIs) are located along one isomerization channel and one dissociation channel. Through the isomerization channel, TMS-CBD is transformed to tetrakis(trimethylsilyl)-tetrahedrane (TMS-THD), and this isomerization process could take place by passing through a tetra form conical intersection. On the other hand, the dissociation channel yielding two bis(trimethylsilyl)-acetylene (TMS-Ac) molecules through further stretching of the longer CC bonds might be more competitive than the isomerization channel after excitation into S3 state. This mechanistic picture is in good agreement with recently reported experimental observations.