Abstract: Chemical looping combustion of coal is one of the most vital technologies for clean coal combustion. A metal-based oxygen carrier in chemical looping process is applied to avoid direct contact between the coal and air,which can eliminate the production NO_x pollutants and improve energy conversion efficiency. In general,there are two reaction pathways for chemical looping combustion of coal: in situ gasification chemical looping combustion(iG-CLC) and chemical looping with oxygen uncoupling(CLOU). The difference in combustion pathway will greatly affect the composition and structure design of oxygen carriers. The research progress of solid metal-based oxygen carriers in coal chemical looping combustion from 2015 to 2020 was discussed in detail,including iron-based,manganese-based,copper-based,nickel-based,calcium sulfate,and other composite oxygen carriers. The advantages and disadvantages of different metal oxygen carriers,reaction path,gas-solid and solid-solid reaction mechanism,interaction between metal and carrier and deactivation principle of oxygen carrier were summarized. Iron-based oxygen carriers are widely applied in iG-CLC system,but the reaction rate of single iron-based oxygen carrier is low. The addition of alkali and/or alkaline-earth metals can improve the reactivity of the iron-based oxygen carriers. on the one hand,manganese-based oxygen carrier can release gaseous oxygen in high temperature and anoxic atmosphere;and on the other hand,the manganese-based oxygen carrier can oxidize reducing gas such as CO and CH_4. By applying inert support materials and alkali metal promoters,the mechanic strength and oxygen decoupling ability can be further boosted. Copper-based oxygen carrier has attracted much attention due to good oxygen uncoupling behavior. However,the deactivation of oxygen carriers due to metal aggregation caused by low melting point of copper and its oxides needs to be overcome. Besides,several research works observed that using iron,manganese and/or copper ore as oxygen carriers can improve the reactivity. The reactivity of calcium sulfate oxygen carrier is excellent,but the possible side-products in coal chemical looping combustion such as sulfur dioxide and hydrogen sulfide need to be addressed.Nickel-based oxygen carrier has good reaction performance in chemical looping combustion of coal,but there are only a few recent studies because of the disadvantages of nickel-based oxygen carrier,such as sulfur poisoning,comparably higher cost,and environmental-unfriendly. Novel bimetallic and multi-metallic oxygen carriers can combine the reaction characteristics of the two metals at the same time,thus significantly improving the overall reactivity of the oxygen carriers. Based on the recent advances of oxygen carrier research,this review suggests four possible areas to explore in the future: 1,combining iG-CLC and CLOU pathways to design novel chemical looping assisted by oxygen uncoupling(CLOU) carriers; 2,developing oxygen carriers with novel support material and metal composition; 3,utilizing metallurgical waste as raw material to synthesize oxygen carriers; 4,designing oxygen carriers with special structures,including coreshell and skeleton structures.