The concept of improving the safety of rechargeable lithium sulfur batteries by introducing flame retardant additives (FRs) to liquid electrolytes is primarily reported in this paper. Dimethyl methylphosphonate (DMMP) is used as a flame retardant additive to improve the thermal safety of rechargeable lithium batteries with sulfur based composite cathode materials. The electrochemical compatibility between DMMP and sulfur cathode is investigated. With 11 wt.% DMMP addition, the electrolyte of 1M LiPF6/EC+EMC (1:1, v/v) is nearly nonflammable and its thermal stability is obviously improved. Cycle performance and electrochemical stability are little affected with an appropriate DMMP addition of 7–11 wt.% by the analysis of electrochemical impedance spectra (EIS) and cycle performances. DMMP is a feasible choice as flame retardant additive in carbonate electrolyte for rechargeable lithium batteries with sulfur composite cathode materials.