Bio-SRF (Bio-Solid Refuse Fuel) based on livestock waste has a low heating value and high moisture content. The concentration of toxic gas, such as SOx, NOx, and HCl, in the flue gas is changed according to the composition of fuel, which has been reported. Therefore, the study of fuel combustion characteristics is necessary. Additionally, the study of fuel firing characteristics is necessary. In this study, we investigated the combustion characteristics of the mixed firing of coal and Bio-SRF made from livestock waste in a circulating fluidized bed combustor (CFBC). The Bio-SRF of livestock waste was mixed with different ratios of coal based on the heating values when the coal was completely combusted in CFBC. In the results of the experiment, the combustor efficiencies of the calculated unburned carbon concentration in the fly ash were 98.87%, 99.04%, 99.64%, and 99.71% when the multi-firing ratio of livestock sludge increased from 100/0 to 70/30. In addition, the boiler efficiencies were 86.23%, 86.30, 87.24, and 87.27%. Through the experimental results, we identified that the mixed combustion of livestock sludge is not affected by boiler efficiency. We have systematically investigated and discussed the temperature changes of an internal combustor, compositions of flue gases, solid ash characteristics, and the combustion and boiler efficiency during the mixed firing of coal and Bio-SRF.