Abstract: Blade twist angle is an important parameter characterizing aerodynamic performance and structural status of helicopter rotors. To address the problem that reconstruction errors at azimuths near the field-of-view boundary tend to spread to other azimuths through global fitting, thereby degrading measurement consistency in conventional methods, a polar-coordinate-mapping-based method was proposed for measuring blade twist angle of coaxial dual-rotor helicopter. First, under a low-speed flattened condition, a local reference plane was independently fitted for each rotational azimuth and a polar coordinate system was established, making measurements at different azimuths mutually independent and preventing edge-azimuth errors from spreading to other azimuths. Second, to cope with flapping-induced marker non-coplanarity under local reference-plane framework, marker points were orthogonally projected onto their corresponding local planes under high-speed rotation, and twist angle was extracted from the change in polar angle of projected points. Experimental results showed that, under dynamic measurement conditions, the mean absolute error across azimuths is less than 0.14°, the standard deviation is less than 0.13°. Compared with conventional methods, both measurement consistency and accuracy were improved, providing a reliable technical means for performance evaluation and structural optimization of coaxial dual-rotor helicopters.