CaSO4 is an interesting alternative oxygen carrier for chemical looping combustion (CLC). Utilization of a CaSO4 oxygen carrier suffers deactivation caused by sulfur loss. To prevent sulfur loss, a small amount of CaO particles were mixed with the CaSO4 oxygen carrier. The predominant regions of CaSO4, CaS, and CaO have been obtained with consideration of SO2 and COS emissions. The predominant region of CaO is increasing with the temperature and CO2 partial pressures. Isothermal as well as nonisothermal kinetics of CaSO4-based oxygen carrier reduction, and reduction/oxidation cycle tests, have been carried out. The effects of reaction temperature, CO concentration, and molar ratio of CaO to CaSO4 and cycle number on sulfur emission and CO2 generation efficiency are taken into account. XRD, XRF, and gas analyses were performed to investigate the variations of solid phase change, elements composition in solid residual, and sulfur release with reaction time. The results show that the use of fresh CaO additive not only enhances CO2 yield but also captures SO2 and COS, which are identified by XRD and XRF analysis. During cycle tests, the use of CaO additive also improves CO2 yield. CO2 yields drop a little after the first cycle, but they can maintain at high levels subsequently in cases with the use of CaO additive. SO2 released from the fuel reactor (SO2-FR) is dominant over COS released from the fuel reactor (COS-FR) and SO2 released from the air reactor (SO2-AR). With the use of CaO additive, the releases of COS-FR and SO2-AR decrease, but SO2-FR emission increases a little. The comprehensive roles of CaO additive on CO2 generation and gas sulfide releases are evaluated by the environmental factor. The CLC with the CaSO4-CaO oxygen carrier with CO, ensuring a low environmental impact during the early 1-6 cycles, could achieve a high CO2 generating efficiency. In subsequent cycles, there was no obvious drop in the E-factor of SO2.