Abstract: With the development and performance improvement of anti-jamming technology for anti-ship missile, the traditional on-platform shipboard jamming technology has not been sufficient to fully resist various naval war threats, therefore as an off-platform jamming method with networking capabilities, outboard active decoy has attracted much attention from navies of various countries. Firstly, the jamming mechanism and working characteristics of outboard active decoy are introduced, and the jamming strategy of using outboard active decoys in the ship defense system is analyzed in this paper, and the effectiveness of outboard active decoy is demonstrated too. Then, on this basis, the developments and technical characteristics of foreign outboard active decoy equipment are clarified. Finally, the countermeasures that can be adopted are analyzed for the anti-outboard interference measures of the new anti-ship missile seeker. The research content gives a comprehensive introduction for the outboard active decoy and its countermeasures, which can provide support for the subsequent design of outboard jamming products and anti-jamming technology of seeker.