The invention belongs to mechanical methods, in particular to a roundness detecting method for an eccentric shaft part. The method is characterized in that a turntable and a measurement support are arranged on a base, an eccentric disc is arranged on the measurement support, a telescopic supporting rod is fixed on the eccentric disc through a rotating bearing, a V-shaped reference block is arranged on the telescopic supporting rod, the telescopic supporting rod is fixed on the eccentric disc through the rotating bearing, and the telescopic supporting rod can freely rotate around the rotating bearing. When the eccentric shaft part is driven to rotate by the turntable, the eccentric disc and the turntable rotate synchronously, so that a measurement probe, the V-shaped reference block and the telescopic supporting rod in the roundness detecting process only move in parallel; when the turntable continuously rotates for one circle, the roundness of the measurement probe on the surface of the part in a 360-degree range is measured by a measurement and control system; and the method is simple, convenient, feasible, scientific, reasonable and high in measurement precision.