Chemical looping gasification (CLG) is a clean and efficient technology that uses oxygen carriers (OCs) to convert carbon energy. Industrial waste phosphogypsum (PG) as raw material for OCs has been proven to be a feasible strategy. Single PG for the preparation of OCs has certain limitations; however, two or more types of elements to modified PG has not been applied in biomass CLG process. In this work, PG was used as carrier and modified with iron and cerium for syngas production through CLG. The PG prepared OCs significantly reduce the costs and improve the utilization rate of solid waste. The reaction conditions of the OCs for syngas production were investigated in a batch fluidized bed. Experimental results show that the best syngas performance is achieved when the PG content is 20 wt.%, Fe/Ce = 2:1, gasification temperature of 800 °C and steam injection rate = 40 μL/min. Multiple redox manifest that the additives of Fe2O3 and CeO2 in the prepared PG carriers increase reaction activity and its stability can be kept at a high level. Finally, the reaction mechanism of modified PG used in biomass CLG was analyzed by combining various characterization methods and density functional theory (DFT) simulations. The possible mechanism suggests that variations of syngas are affected by the calcium roles and valence change of iron. This research provides an effective pathway for the conversion of biomass and the utilization of solid waste PG.