The present invention relates to gaseous discharge and applied biotechnology. In particularly, the present invention relates to a method for killing organisms in the course of conveying ballast water in ship, comprising applying a voltage with frequency of 400-40000 Hz between the anode electrode and the cathode electrode of the free radical generator, thereby forming strong ionization discharge of electric field strength of 250-600 Td between the anode electrode and the cathode electrode, ionizing, dissociating, and exciting O.sub.2 and H.sub.2O, or O.sub.2, processing into active particles comprising O.sub.3, OH. and HO.sub.2, dissolving excitedly to obtain free radical solution mainly consisted of OH, adding the free radical solution to conveying pipeline of ballast water, conveying for 2-20 s in the pipeline, thereby killing organisms of ballast water selected from the group consisted of bacteria, single cell alga, and protozoan.