A quaternary ammonium salts (QAS) was synthesized from trialkylamine reacting with a halocarbon compound. This QAS was characterized by analytical methods of precipitation and spectroscopic determinations. QAScan damage marine life by killing one-celled plants to destroy their growth systems, and the prepared QAS therefore was used in biocidal experiments for examining their biocidal efficiency towards several bacteria and marine planktonic algas. When the concentrations of the prepared QAS reached at 3.2 and 50 ppm, growth system of the selected bacteria and alga were obviously inhibited respectively.