A semiconductor laser bar with low series resistance and relatively high internal quantum efficiency is designed for low-temperature operation. The structure is optimized elaborately with a low Al content Al_xGa_1-xAs in the waveguide. The peak electro-optical conversion efficiency of the optimized structure increases to 82.3% at -65 ℃. The constituents of power loss at the peak of the efficiency under different temperatures were also studied. It is observed that the main factor affecting the electro-optical conversion efficiency at low temperatures is the series resistance. By continuously reducing the device resistance, high efficiency at low temperature will be obtained if carrier leakage and optical loss can be controlled.