<p>The vortex settling basin for suspended sediment is a sediment treatment device that enables efficient use of irrigation water, yet the diameter design remains insufficiently defined, leading to large dis crepancies between measured and designed trapping efficiency in practice. This study proposes a diameter design method based on the tangential characteristic Reynolds number. Field observations were first conducted at three representative projects—Donglei, Talan River, and Shangmaxiangdi. Guided by these observations, laboratory physical models were developed, and an Acoustic Doppler Velocimeter was used to measure and analyze the distribution of tangential characteristic Reynolds number in each basin. The results show that when the sediment trapping efficiency is high, the tangential characteristic Reynolds number should be less than 9.5 × 10⁶ for radial positions between 0.80 and 0.92 at the 180-degree section or between 0.60 and 0.92 at the 270-degree section, where the radial position is the distance from the measurement point to the basin center divided by the basin radius. Building on this relationship, a diameter design method for the vortex settling basin for suspended sediment is proposed. Verification using prototype projects indicates that replacing the traditional empirical approach with the proposed method can increase the trapping efficiency of suspended sediment by about 9.4%–21%. These findings provide practical guidance for diameter design.</p>