Biosurfactant is a kind of amphoteric compounds deriving from organisms with both hydrophilic group and hydrophobic group.It has been applied in various fields, such as in the regulation and control of the harmful algae blooms (HABs).However, surfactant secreted from the marine microorganisms is rarely studied.In this research, we chose a typical red-tide dinoflagellate--Scrippsiella trochoidea as the aimed alga and strain Halomonas sp.DHQ19, which has good emulsifying activity and algal inhibition activity, was further studied.