In view of the intractable use of wet?process phosphoric?acid slag and phosphate tailing，a strategy was proposed for preparing calcium magnesium polyphosphate fertilizer via calcining wet?process phosphoric?acid slag and phosphate tailing composites.Based on the decomposition ratio of phosphate tailing，the suitable mass ratio of phosphate tailing and wet?process phosphoric?acid slag was 0.30，and the decomposition ratio was 80.18%.The results of calcination process showed that both increasing temperature and prolonging time promoted the growth of polyphosphate chain.The average?degree of polymerization was increased from 1.00 to 3.23 after calcinating at 105~460 ℃ for 1.0 h.Among them，at 340 ℃ the polymerization rate reached 85%，but the polymerization degree was still increasing.Although the water solubility of fertilizer decreased gradually with the increase of polymerization chain，the available phosphorus content was still more than 90% when the temperature did not exceed 340 ℃.After calcination at 340 ℃ for 1.0~2.0 h，it was found that when calcination time was 1.0 h，the polymerization rate and average degree of polymerization reached 87.4% and 2.80，respectively.Optimizing the processing parameters，the polycondensation could be controlled on the level of polymerization degree distribution，which opened a new direction for the construction of slow-release fertilizer.Polyphosphate containing species with polymerization degree from 1 to 10 could be optimized under the calcination of 340 ℃ for 1.0 h，which resulted with amorphous and porous-structure.The available phosphorus，calcium and magnesium were over 85%（mass fraction），which could be used as a new multi?element slow?release fertilizer.