The growth and physiology of Chlorella pyrenoidesa were studied by using different concentrations of G. pentaphyllum extract (0, 5, 10, 25, 50 g/L). The results showed that: (1) G. pentaphyllum extract could inhibit the growth of C. pyrenoidesa, with the increasing concentration and the prolonging time of cultivate, the inhibition increased. And the inhibition rate reached the maximum that was 79.41% when C. pyrenoidesa was cultured for 15 days with 25 g/L of G. pentaphyllum extract. (2) Chlorophyll a content in the cells of each concentration treatment group was lower than that of control group. With the increase of extraction concentration and the extension of treatment time, chlorophyll a content decreased more than that of control group. This result indicated that the photosynthetic effect of C. pyrenoidesa was affected. (3) The cell membrane permeability (absorbance OD₂₆₄) of the treatment groups was significantly higher than that of control group, which meant the cell membrane permeability increased with the increase of the concentration of the extract. At high concentration, both OD₂₆₀ and OD₂₈₀ were significantly higher than that of control group. And the cell membrane permeability was increased with the extension time, and more soluble proteins and nucleic acid were infiltrate out of the cells. The experimental results indicated that inhibition was enhanced with the increase of the concentration, and its mechanism of action was to promote chlorophyll decomposition and increase cell membrane permeability as well as promote soluble protein and nucleic acid exosmosis. So the structure of algal cells damaged and metabolic disorders happened, thus allelopathic inhibition was achieved.