A kind of aqueous extracts of Porphyra yezoensis was extracted with distilled water, and its inhibitory activity against four species of red tide microalgae, Amphidinium hoefleri, Karenia mikimitoi, Skeletonema costatum and Alexandrium tamarense was tested. The extracts were extracted with five solvents (methanol, acetone, ethyl acetate, chloroform and petroleum ether), respectively, to probe their inhibitory effects on the growth of those microalgae. Based on the observation of microalgal morphology and density, cell size and the contents of some key physiological indicators (chlorophyll, proteins and polysaccharides), the inhibitory effect of the solvent extract that had strongest inhibition on all tested microalgae was investigated. Thereafter, the extracts mentioned above were separated again with petroleum ether phase, ethyl acetate phase, nbutanol phase and distilled water phase by the method of liquidliquid fractionaction. The results showed that the growth of the four microalgae was significantly inhibited by the aqueous extracts of P. yezoensis at the concentration of 16 g/L. The inhibitory effect of the aqueous extracts of P.yezoensis for A. hoefleri and K.mikimitoi was stronger than that for A.tamarense and S.costatum. The methanol extracts of P.yezoensis dry powder had the strongest effect, and at the concentration of 16 g/L its growth inhibitory effect on A. hoefleri, K.mikimitoi, A.tamarense and S.costatum was 70.5%, 79.9%, 67.1% and 65.1% on day 12, respectively. At the same time, the cell size and athletic ability of the cells decreased, and cavities, pieces and pigment faded in cells were caused. The further investigation found that under the effect of the methanol extracts the contents of chlorophyll, proteins and polysaccharides in the cells of those microalgae significantly decreased. The methanol extracts were partitioned to petroleum ether phase, ethyl acetate phase, nbutanol phase and distilled water phase by liquidliquid fractionaction, among which those extracted with petroleum ether and ethyl acetate had the strongest effect.