Phosphorus tailing mainly comes from the rest of the mineral extraction concentratelater slag, it belongs to the tail of industrial waste solid waste.And the environmental would be hamed and the natural resources would be wasted if processing and utilization of phosphorus tailing is not optimistic. The thermal research separation magnesia and optimum separation conditions of CaO and MgO of the phosphorus tailing were determined.When the first samples at different temperatures calcined different time, the completely thermal decomposition temperature should be 950±10 ℃, the temperature range was 920～950 ℃.The best conditions to divide CaO and MgO were the temperature of 800 ℃ and the decomposition time of 5 h through testing the product activity and carbonization method.