A system for decreasing volatile organic compounds (VOC) in a crude oil tanker includes: a supply pipe horizontally installed over a crude oil storage tank of the crude oil tanker and having an end connected to a crude oil storage tank on land; a load pipe vertically connected to a bottom surface of the supply pipe, vertically passing through the crude oil storage tank of the crude oil tanker, and communicating with a lower part of the crude oil storage tank; a distribution pipe horizontally connected to a lower end of the load pipe; and a pressure control unit installed at a lower end portion of the load pipe.