In order to achieve rapid and high-precision measurement of dissolved oxygen mass concentration using optical methods, a dissolved oxygen measurement method based on sampling-average to detect phase difference has been proposed. The multi-sampling average method was used to eliminate single-point error and improve measurement accuracy. Theoretical analysis and experimental validation were conducted. The measurement accuracy, measurement speed, and the influence of sampling frequency on the measurement error were studied. Comparative assessments were made against the performance of various existing methods, and a dissolved oxygen measurement system was set up to validate measurement results. The results indicate a clear advantage ofusing the sampling-average method to detect phase difference, with high measurement accuracy (measurement error is 1.13°, measurement standard deviation is 1.24°) and fast measurement speed. The relative measurement error of the dissolved oxygen measurement system is 2.84%, and the relative measurement standard deviation is 4.51%, achieving high-precision measurement of dissolved oxygen mass concentration. This research provides a simpler and more accurate phase difference detection method for high-precision dissolved oxygen measurement using optical methods.