Abstract: By using water dissolution to obtain potassium contained solution with straw-ash of power station as the raw material and then the potassium and sodium were separated from the solution through extraction method.Finally，potassium-magnesium sulphate fertilizer was successfully synthesized.The optimal technical parameters of water dissolution were determined by experiment as follows: the mass ratio of water to ash was 4∶1，the extracting temperature was 60 ℃，and the extracting time was 60 min，and then the potassium extracting efficiency was 80.74% under those conditions.The best technical constants of extraction method were got through orthogonal test and were given below：the volume ratio of tributylamine to n-butanol was 1∶2.5；the volume ratio of organic phase（extract agent） to inorganic phase（potassium contained solution） was determined as 1∶2；the extracting time was 2 h，and the extracting temperature was 60 ℃.The measured results showed that the sample accorded with the national standard of potassium magnesium fertilizer and had a higher level than superior product.

Key words: straw-ash, potassium-magnesium sulphate fertilizer, phase separation, orthogonal test