Abstract: The optimal proportion in the process of producing building brick from modified phosphogypsum was studied. Through the low-temperature ceramics modification of phosphogypsum using composite cementitious material, the optimal proportion for DSZ: FMH: KZ is obtained to be 1:1:1 with orthogonal experiments firstly. Then the best ratio of phosphogypsum to composite cementitious material is 1:1. Lastly with the constant phosphogypsum content of 50%, the best content of DSZ, FMH and KZ is 12%, 18% and 20% respectively and the best curing time is 20 h through single factor experiments.