Dimethyl sulfite (DMS) is studied as an additive in lithium bis(oxalate)borate (LiBOB)/γ-Butyrolacton(GBL) electrolyte with LiCoO2 cathode and mesocarbon microbeads (MCMB) anode. Electrochemical performances of LiCoO2/DMS-electrolyte/Li and Li/DMS-electrolyte/MCMB half cells are studied by cyclic voltammetry, battery cycler, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thermal behaviors of electrode–electrolyte coexisting systems are evaluated using a C80 micro-calorimeter. The electrochemical results show that the polarization of LiCoO2 is decreased and the capacity is improved with the addition of proper content of DMS. The XPS and SEM examination show that the presence of DMS decreases the decomposition of BOB anion and modifies the formation of SEI film on the surface of MCMB anode. The C80 micro-calorimeter analysis shows that the electrode materials with a LiBOB/GBL+proper DMS electrolyte begin to release heat at higher temperature and generate less heat than that with a common electrolyte. Therefore, using proper content (such as 20%) of DMS as an additive in LiBOB/GBL electrolyte is beneficial for the performance of Li-ion batteries.