Abstract: The purpose of this experiment is to study phosphorus and potassium interaction effect of four Bacillus strains A, B, C and D, find out the best combination of strains, and provide technical support for the development of compound fertilizer. Molybdenum antimony colorimetric method and flame spectrophotometer method were employed to determine the soluble phosphorus, potassium ability, solution pH value and total content of organic acid changes in culture and the relationship between each other of individual and mix-cultured strains of 4 Bacillus strains. The results showed that in the culture fermentation supernatant of inorganic phosphorus, soluble phosphorus content of mixed strains was significantly higher than that of single strain, soluble phosphorus content had a significant negative correlation with the pH, and an obviously positive correlation with the total content of organic acid. In organic phosphorus fermentation supernatant, the processing difference of soluble phosphorus content was not significant (P>0.05), there was no significant correlation between soluble phosphorus content and pH and total organic acid. In the potassium ore fermentation culture supernatant, each treatment group had certain ability of dissolving potassium, and the contents of soluble potassium had certain correlations with its pH and total organic acid, while total organic acid amount had a certain correlation with and the pH. But the difference of soluble potassium content was not significant (P>0.05). Mix-cultured strains, especially three and four strains combination treatments could significantly improve the soluble phosphorus content, but did not fully show the bonus effect. Considering the characteristics of each treatment, the 13th group had strong synergistic effect, and could become the best combination of strains.