Abstract: Flue gas desulphurized slag was collected as the raw material and purified using the multiple steps of sour purification procedures, which was further prepared as CaSO₄ oxygen carrier (OC). Reaction characteristics of the purified desulphurized slag OC with the selected lignite as well as release of the different gaseous sulfur species from the CaSO₄ side reactions were investigated on in a lab-scale fixed bed reactor. Several main influencing factors, including reaction temperature, OC excess number Φ and the cycle numbers, were focused. The experimental results indicated that the purified CaSO₄ OC was of high reactivity and promising to be applied in the coal-fueled chemical looping combustion process as OC. Furthermore, according to the pros and cons of the three influencing factors on the carbon conversion versus the gaseous sulfur release, the optimized reaction conditions were determined as 900℃ and the OC excess number around 1.0. Finally, under this optimized reaction condition, the effect of the cycle numbers was evaluated and found that the continuous release of gaseous sulfur from the CaSO₄ side reactions deteriorated the reactivity of the purified CaSO₄ OC and its reaction stability was diminished with the five cycles.