With the seafloor observatory networks long-term，continuous，long-distance power supply characteristics，the adoption of monopolar direct current（DC）transmission mode has become a trend.The use of a length of copper wire of transmission from optic/electric composite cable and the seawater as transmission circuit mode has reduced not only the cost but also the transmission loop loss.The key technology of the seawater as a conductor in monopolar DC transmission mode is analyzed.The main influencing factors of seawater loop resistance are verified by experiment.The potential effect of seawater as a conductor on the marine environment，marine life and human security are dealt with.