Abstract: Nowadays, the marine environment is deteriorating and red tides occur frequently. Algacidal bacteria is one of the efficient ways to inhibit red tides, however, there is still limited research on the algacidal bacteria. In order to alleviate the harmful effects of red tide microalgae on the marine ecological environment, we screened algacidal bacteria from mud samples of intertidal zone, and carried out biological characterization. Skeletonema costatum and Prorocentrum donghaiense were chosen to evaluate the algacidal efficiency. extracted by the acetone method and the chlorophyll content was measured. A total of 43 strains were obtained from primary screening, and the strain which showed best algacidal effect was identified as Bacillus by morphological observation, physiological and biochemical characteristics and 16S rDNA sequence analysis, and was named as Bacillus sp. 2-1-2. The supernatant of bacteria solution showed strong algacidal effect while the resuspension of bacteria showed no or less algacidal effect, indicating an indirect algacidal mechanism. After coculture of the supernatant of bacteria solution with algae for 30min, both ROS and MDA levels of the algae changed very significantly (P<0.01). It is speculated that the mechanism of algae lysis may be the secretion of algae lysis active substances indirectly contacting the algal cells, affecting the normal physiological state of the algal cells, stressing the imbalance of the dynamic oxidation balance of the algal cells, the lipid peroxidation of the algal cells destroying the structural integrity and causing the contents to leak out and gradually break down, inhibiting the growth of algal cells. Different conditions, including pH, fermentation time and adding ratio, were studied to ensure their influence on the algacidal effect of the bacteria solution. The results showed that this strain was not tolerated to acidic nor basic condition. The optimum fermentation time was 2—4d and the optimum addition rate of bacteria solution was 20%. The properties of algacidal substance were preliminary investigated using the supernatant of the bacteria solution. It is notable that the algacidal substance was resistant to heat, basic condition and weak acidic condition. This work provides theoretical basis for the deeper research at the molecular level and further utilization of algicidal bacteria.