The blade flaps during the rotation reflect the performance of the helicopter rotor. The measurement and analysis of the blade flaps in the whole rotation field is a difficult problem in the helicopter rotor test. To address this issue, a new method is proposed, which is based on stereo vision with large field of view ( FOV). Firstly, a stereo vision system with large FOV is constructed and the 3D coordinates of the markers on the blades are measured. Secondly, the blade flaps of the markers are resolved in the hub coordinate system. Finally, the flapping modes of the blade at each azimuth are obtained by using the fourth-order polynomial regression model. The flapping law of the blade during the rotation is achieved by utilizing the regression model of compound sine functions. The proposed method is evaluated by the measurement experiments on the blade flaps of the hovering helicopter. In the 4. 6m × 4. 6m scene, the root mean square error of the blade flap measurement is less than 1mm, and the regression models of the flap mode fit the blade flap data with the root mean square error less than 1mm.