Abstract: Bacteria were isolated and purified from Taihu Lake, where algal blooms occur annually. We used LB liquid and solid slant pure culture solutions of bacteria to infect Microcystis aeruginosa cells, which resulted in the identification of seven strains of bacteria with algae-lysing activity. The method by which the algae-lysing bacterium THW7 lyses algae and the effects of its algae-lysing active substances on the physiological activities of Microcystis aeruginosa were preliminarily studied. The results showed that there was a risk of failed screening or overestimating the efficiency of algae-lysing bacteria when using only LB liquid pure culture solution to screen algae-lysing bacteria, and this risk could be avoided by using the solid slant pure culture solution of bacteria. The algae-lysing bacterium THW7 indirectly lysed algae by secreting extracellular active substances. When treated with the aseptic filtrate of THW7, the growth of Microcystis aeruginosa was significantly inhibited (P<0.01), and the algae-lysing efficiency reached 94.38% on the 10th day. Additionally, the activity of the photosynthetic system significantly decreased (P<0.01); the MDA content accumulated; and the activities of SOD, POD and CAT generally increased first and then decreased and were significantly higher in the treatment group than that in the control group (P<0.01). The algae-lysing active substances secreted by strain THW7 may act on photosynthetic system II of Microcystis aeruginosa cells, thus preventing electron transfer and inhibiting the photosynthetic process. These substances also induced oxidative damage in the algal cells and destroyed the integrity of the algal cell membrane to achieve the algae-lysing effect.