The invention discloses a cross-sea long-distance angle measurement method and a cross-sea long-distance angle measurement system. The method comprises the steps of analyzing a measurement result of each observation set and judging whether the measurement result meets the measurement requirements; adding the observation sets and screening more accurate measurement results in the measurement process; and repeating the previous steps until all measurement results meet the measurement requirements. Therefore, the influence of an unstable offshore measurement platform on long-distance angle measurement can be overcome; the time spent for the whole long-distance angle measurement is shortened; the measurement efficiency is greatly improved.