Abstract: In this study,one high-effect algae-lysing bacterium was isolated and purified from Lake Dianchi,which wasabled as the strain DC-L5. The effect,host range,biosafety,lysing mechanism and lysing phenomenon of the algae-ly-ing bacteria were analyzed. Resultswere as the followings: The strain was identified asB acillus pum ilus bymeans ofmor-hological observation and 16S rDNA sequence analysis in the CCTCC (China Center for Type Culture Collection). Toxic-ty test with mouse primarily showed that the strain was non-toxic. 4 kinds of DC-L5 cultures of different growth phasesere added to M icrocystis aeruginosa,and the strongest algae-lytic ability was found in the log phase of DC-L5 cultures,hlorophyll a contents in culture ofM icrocystis aeruginosa reduced by 83. 33% in 5 days. For the host range experiment,kinds of dominant cyanobacteria in Lake Dianchi was researched,and average reduction rates of Chlorophyll a of-. -5% in cultures ofM icrocystis viridis,M icrocystis wesenbergii,Aphanizomenon flos-aquae and Anabaena flos-aquaewasbserved with the highest 67. 6% and the lowest 58. 5% in 4 days respectively. After centrifugation and sedimentation,oth the cells of bacteria and the cell-free suspension had strong algae-lytic activity,but the bacteria culture had the stron-est algae-lytic activity,so it maybe showed the strain DC-L5 could directly cause the cyanobacterial cells agglomerated,inked down and further lysed while the strain could excrete extra-cellular substances able to lyse cyanobacterial cells. Inrder to improve the algae-lytic activity of the strain DC-L5 to nature bloom,the bacteria cultures were added to natureyanobacterial bloom dominant with M icrocystis aeruginosa,which were samp led from Lake Dianchi and cultured in theab. The result of nature bloom experiment showed the strain DC-L5 firstly disaggregated the cyanobacterial colony,educed the floatability,degraded the p igments and then released inclusions of algal cells.