Abstract: Preset undersea weapon is regarded as a new form of future undersea attack and defense weapon equipment. Variety of payloads such as unmanned aerial vehicle(UAV), unmanned undersea vehicle(UUV), missile, torpedo which are used to perform tasks like reconnaissance, strike, area denial, etc., can be preset on a continental shelf or an island chain through preset undersea weapons for long time lurk, waiting for remote activation. This system aims to construct the multi-dimensional battlefield by taking the advantage of integrated resources of the land, maritime, undersea, air, space. In this paper, the characteristics, functions and key technologies of the preset undersea weapon are analyzed according to recent open reports and literatures. Eight key technologies are summarized, including remote activation technology, deep-sea long-term anti-pressure and corrosion protection technology, preset load launching technology, stealthy deployment technology, deep-sea detection and reconnaissance technology, underwater networking and communication technology, deep-sea energy self-sustaining and resupply technology, and auxiliary decision making technology. This article may benefit to promote the construction of undersea preset unmanned combat equipment in China.