Abstract: The undersea gold deposit off northern Sanshandao is a newly discovered super-large gold deposit in the northern Jiaodong Peninsula, located at the northern end of the Sanshandao- Cangshang Fault Zone. A large amount of research work has been carried out on the shallow part of this deposit, but the research on the deep mineralization laws and resource potential is relatively weak. The systematic mineralogical study on pyrite of the deep part of this deposit (at an elevation of –1253 to –1455m) is conducted in this paper, to reveal its characteristics of geochemical compositions, explore the genesis of the deposit, evaluate the potential for deep mineral exploration, and provide a scientific basis for regional deep mineral exploration prediction. Based on the theories of genetic mineralogy, this paper conducts an in-depth study on the morphology and compositional characteristics of pyrites from three metallogenic stagesfrom drill coresin this deposit. We obeserved the morphological, particle size, and self-shaping degree characteristics of pyrite and analyzed in-situ trace element composition of pyrites at each stage, using electron probe technology, with a focus on determining the contents of elements such As Fe, S, As, Co, Ni, and Au, and calculating standard parameters such as Co/Ni value and Fe/(S+As) value. The results indicate that the pyrites in this deposit can be divided into three mineralization stages: pyrite sericilization stage (Py1), quartz-pyrite-arsenopyrite stage (Py2), and quartz-polymetallic sulfide stage (Py3). Among them, the pyrites of stage Py2 and Py3 have smaller particle size and poorer hedral degree which have better gold content. he Co/Ni ratio of pyrites in this deposit ranges from 1 to 4, with an average of 3. The pyrites are rich in As and Co, which further indicate that this deposit is of magmatic hydrothermal origin. Collectively, Fe/(S+As) ratios of pyrites, alongside the vertical zonation characteristics of trace element, indicate high prospectivity at depth in the undersea gold deposit off northern Sanshandao.