Abstract: K ⁺，Mg^{2 +} ∥ SO₄^2-，Cl^-—H₂O quaternary system phase diagram analysis was used in this study.The experiments showed that particle size of potassium magnesium fertilizer product transformed from kainite was improved obviously at 55 ℃.The influences of the amount and particle size of crystal seed on transformation particle size were studied.The problem of too fine of potassium magnesium fertilizer powder was solved and dust pollution in the production process was reduced to a certain extent because of it.Environmental protection and better resource utilization rate were achieved at same time， reducing the waste of resources and providing experimental basis for potassium magnesium fertilizer production process improvement.

Key words: potassium magnesium fertilizer, phase diagram, crystal seed, particle size, dust