The invention belongs to the field of mechanical methods and particularly to a method for measuring the roundness of an eccentric part. The method is characterized in that the eccentric part is installed on a rotating table, a V-shaped reference block is installed on a telescopic support rod, the support rod is mounted on a reference frame having a determined distance to the center of the rotating table, and the support rod can rotate freely within a certain angle range around the reference frame; a measuring probe is mounted on the V-shaped reference block, and a signal of the measuring probe is connected to a measurement and control system; when the eccentric part revolves under the drive of the rotating table, two reference surfaces of the V-shaped reference block can be always kept in externally tangent relation with the eccentric part under the action of elastic part of the telescopic support rod; when the eccentric part continuously revolves for one circle, the measuring probe mounted on the V-shaped reference block continuously goes around the surface of the part for one circle, the measurement and control system can acquire a fluctuation signal related to the roundness of the measured eccentric part and further get the roundness of the surface of the eccentric part within a 360-degree range. The method is simple and easy to operate, scientific and reasonable, and has high measurement precision.