Mechanisms and channels of the C2H5O2 + ClO reaction in atmosphere were investigated using quantum chemistry method, i.e., CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(d,p) levels of theory. Our result indicates that the title reaction occurs on both the singlet and triplet potential energy surfaces (PESs). Kinetically, C2H5O3Cl and C2H5O + ClOO on the singlet PES were dominant in atmosphere; while other products including CH3CHO2 + HOCl, CH3CHO + HOClO, and CH3CHO + HClO2 make minor contributions to overall reaction due to high barriers and unstable formations. Moreover, time-dependent density functional theory (TDDFT) calculation on the vertical excitation energy implies that C2H5O3Cl will photolyze under the sunlight.