Though there have been many empirical formulas about the required width of navigation in curved channel, the drift-angle involved in the formulas is measured from tests. The main factors affecting the drift angle, such as R/L (R is the radius of curvature, and L is the ship length), flow velocity, navigation speed and maneuverability, are analyzed through ship model tests in curved channel. According to dimensional analysis and linear-regression analysis, the formula of drift-angle in curved channel is put forward. The calculated results are compared with data from ship model test and real ship test, and are validated to be consistent.