A measurement method based on image is present to measure rotation angle at high precision of 5″ which uses a line as the character sign. A digital camera is used to capture the image of the line. The rotation angle is calculated by detecting the line slope angle with high accuracy in the image. The factors that affect the precision are analyzed. Then the guidelines to measure a rotation angle with high precision of seconds are proposed, which include the method to detect a line and the parameters of a line such as the length,the width and the direction. Experiments show that when the line is wide of 5～9 pixel and the angle between the line and the sampling direction is far away from 0° and 45° the rotation angle measurement gets the highest accuracy, and if the length is not less than 1000 pixel the precision is not less than 5″.