A method for rapidly measuring the roller pass major semi-axis of a three-roller reducing mill to improve the outer-diameter precision and surface quality of a steel pipe includes selecting a corresponding value of the average diameter of an elliptical pass, a value of a first major semi-axis, and a value of a minor semi-axis according to a rack station number of a special numerical control lathe; calculating a value of a first line segment according to an elliptic equation, the obtained value of the first major semi-axis, and the obtained value of the minor semi-axis; calculating a value of a second major semi-axis according to the right-angle triangle theorem and the value of the first line segment; and measuring a value of the first dimension by attaching three claws of a three-claw internal micrometer to three roller gaps.