The selection of inoculation carrier for functional bacterial fertilizer directly affects the survival and function of functional microorganisms.In this study，the widely used peat was used as the control carrier，and the composite carrier material was prepared by mixing biochar，straw powder and spent mushroom substrate in different mass ratios，and the effects of different carrier materials on the survival rate，survival time and potassium solubilization activity of Bacillus strains were investigated by loading four Bacillus strains with potassium-solubilizing capacity screened before.The results showed that the viable strain number in the two composite carriers [straw powder∶spent mushroom substrate∶biochar(3∶1∶1)and straw powder∶spent mushroom substrate(2∶1)] was higher than that of the two single carriers (peat and biochar) throughout the entire storage phase of the inoculants.Overall，the stage of microbial acclimatization to the carrier material resulted in a significant decrease in viable strain number in all carriers at the beginning of the inoculants storage phase (0-35 d).Subsequently，the viable strain numbers increased by utilizing the nutrients in the carrier for growth and reproduction，but the limited nutrient storage capacity of carriers led to a decrease in viable strain numbers in the later storage stages of the inoculants (75-180 d)，followed by stabilization.The viable numbers of each strain on different carriers showed the trend of “decreasing-increasing-decreasing” with the prolongation of storage time.Among them，the viable strain numbers of the composite carrier materials prepared in the mass ratios of straw powder∶spent mushroom substrate(2∶1)and straw powder∶spent mushroom substrate∶biochar (3∶1∶1)were maintained at more than 108 cfu/g during the storage stage for 180 d，which was in accordance with the technical standards of China’s bio-organic fertilizer industry.The overall potassium-solubilizing activity of each strain on the composite carrier straw powder∶spent mushroom substrate∶biochar(3∶1∶1)was higher than the other three carriers during the 180 d of storage of the inoculants.Influenced by the specie differences of the inoculated strains，the activity of the four Bacillus strains changed differently with the prolongation of the storage time of the carriers，but the change of the potassium-solubilizing activity of each Bacillus strain was basically the same on the four different carriers.The viable strain numbers and activities of the strains during the storage of the inoculants were largely influenced by the nature of the carrier materials.The main manifestations were that the composite carriers of straw powder∶spent mushroom substrate (2∶1)and straw powder∶spent mushroom substrate∶biochar(3∶1∶1)with neutral or weakly alkaline，higher organic carbon content and stronger water-holding capacity were more suitable for the survival and performance of potassium-solubilizing function of Bacillus strains.This study provided new ideas for the preparation of potassium-solubilizing strain inoculants，which could provide theoretical guidance for the development and application of efficient bio-potassium fertilizer in the future.