One of the fundamental photoreactions for ketones is Norrish type I reaction, which has been extensively studied both experimentally and theoretically. Its a bond-cleavage mechanisms are usually explained in an adiabatic picture based on the involved excited-state potential energy surfaces, but scarcely investigated in terms of a non-adiabatic picture. In this work, the S-1 alpha bond-cleavage reactions of CH3OC(O)Cl have been investigated by using the CASSCF and MRCI-SD calculations, and the ab initio based time-dependent quantum wave-packet simulation. The numerical results indicate that the photoinduced dissociation dynamics of CH3OC(O)Cl could exhibit strong nonadiabatic bond-fission characteristics for the S-1 alpha C-Cl bond cleavage, while the dynamics of the S-1 alpha C-O bond cleavage is mainly of adiabatic characteristics. This nonadiabatic mechanism for Norrish type I reaction of CH3OC(O)Cl is uncovered for the first time. The quantum wavepacket dynamics, based on the reduced-dimensional coupled potential energy surfaces, to some extent illustrates the significance of the nonadiabatic effect in the transition-state region on the dynamics of Norrish type I reaction.