Experiments were conducted by compounded various straw biochars and Bacillus mucilaginosus in order to select the optimal carrier to improve the survival rate of Bacillus mucilaginosus, slow its senescence rate, prolong its survival time in soil, and promote the releasing efficiency of soil insoluble potassium. Incubation experiments showed that neutral wheat straw biochar could be used as the ideal carrier for Bacillus mucilaginosus. After 35-d incubation, the content of soil available potassium of biochar-based potassium-releasing fertilizer was 28.53%, 20.75% and 13.41% higher than CK, wheat straw biochar and microbial liquid treaments, respectively. Although the quantities of the Bacillus mucilaginosus both in microbial liquid treatment and biochar-based potassium-released fertilizer treatment were significantly decreased, the decreased rate in biochar- based potassium-released fertilizer treatment (1.0×10⁹ cfu/g to 1.4×10⁷ cfu/g) was lower than microbial liquid treatment (1.1× 10⁹ cfu/g to 9.0×10⁵ cfu/g). Field experiments showed that biochar-based potassium-released fertilizer could significantly increase total potassium contents in tomato plants and fruits, increase the yield and quality of tomato, promote the content of vitamin C, and decrease nitrate content. These results indicate that compound neutral wheat straw biochar and Bacillus mucilaginosus can improve the survival rate of inoculant and promote effectively the release of mineral potassium from soil.