Measuring the detuning angle distribution within the full aperture of a large-aperture potassium dihydrogen phosphate (KDP) crystal is of great engineering importance for improving its frequency conversion efficiency. To improve the measurement speed of the crystal detuning angle, we propose a measurement method based on a flattened Gaussian beam with a large divergence angle. Theoretical simulation shows that the detuning angle of the KDP crystal is linearly related to the vertical position of the maximum light intensity of the second-harmonic laser when using this method. Experimental results confirm our theoretical analysis and show a relative measurement error of 1.32%, which meet the requirements of engineering applications. This measurement method saves 96% of the measurement time of the detuning angle of a single point within the aperture of the KDP crystal, enabling us to measure more points in a limited time, thus improving the measurement accuracy of the detuning angles over the full aperture of the KDP crystals.