Abstract: A positioning method includes the steps of: obtaining a first geographic coordinate set of a first geographic position at water level in a geographic coordinate system; after a positioning system dives into water at the first geographic position, measuring acceleration in the geographic coordinate system for a predetermined time duration; calculating a displacement vector of the positioning system during the predetermined time duration; calculating an estimated coordinate set of an underwater position of the positioning system according to the displacement vector and using the first geographic coordinate set as an initial point; and outputting the estimated coordinate set to indicate the underwater position of the positioning system.