To accurately determine the optical axis cut error of a nonlinear uniaxial crystal, a measurement method based on dual-optical path second-harmonic energy (SHE) rocking curve acquisition is presented in, of which the measurement uncertainty can be controlled within 3.20 µrad , 26 times higher than that of a high-precision commercial x-ray diffractometer (XRD). To meet the measurement requirements, a Type I potassium dihydrogen phosphate reference crystal (RC) is first made, and its optical axis cut error is considered as a reference. Then, the optical axis cut error of a measured crystal (MC) with an aperture of 25mm×25mm and a thickness of 10 mm is determined by simultaneously recording the SHE scatter plots of the RC and the MC, where the measurement repeatability of 10 consecutive measurements is only 4.48 µrad and the measurement speed is within 20 s. During data processing, a third-order Fourier polynomial is proposed to fit the scatter plots into smooth curves, of which the regression coefficients are greater than 0.9975. The experimental method not only overcomes the shortcomings that XRDs will introduce scratches and defects to the crystal surface and are unable to measure large-aperture crystals, but can be used to guide the production of precise cutting of a nonlinear uniaxial crystal, thus ensuring the maximum conversion efficiency of a frequency multiplication system.