As a probiotic widely used in agricultural production，the efficient proliferation of Bacillus subtilis is crucial for the effectiveness of microbial inoculants.This study aimed to clarify the growth-promoting potential of microalgae on B. subtilis and the optimal concentration，screen microalgal strains with significant promoting effects，and analyze their action modes.Using microalgae isolated from extreme environments and B. subtilis as experimental materials，six microalgal strains with significant growth-promoting effects were obtained through primary screening.Identified by 18S rDNA，16S rDNA sequence analysis，the six strains belonged to Chlorella sorokiniana，Anabaena sp.，and Pseudospongiococcum sp.(Note: Pseudospongiococcum is a newly proposed genus).Concentration gradients of 0% (control CK)，20%，40%，60%，80%，and 100% were set up，using live algal culture solutions，supernatants，concentrated solutions，and lysates of microalgae to study their promoting effects on B. subtilis growth.The results showed that the concentrated solution of Chlorella sorokiniana CCAXNU-001 exhibited the most pronounced growth-promoting effect.With the increase in secretory concentration，the promoting effect first increased and then decreased，reaching the optimal effect at 60%，after which the effect weakened.Soluble secretions of microalgae significantly promoted the proliferation of B. subtilis，with the concentrated solution treatment outperforming live algae，supernatants，and lysates.This study clarified the growth-promoting effect and concentration-dependent response of microalgae on B. subtilis，providing a theoretical basis for developing efficient microbial formulations based on microalgae-probiotic combinations.