[Objective] The particle size distribution of fine-grained loess is a key indicator for evaluating its engineering mechanical properties. The traditional hydrometer method is limited by theoretical assumptions, experimental procedures, and dispersion effects, leading to insufficient measurement accuracy and long testing periods, which makes it difficult to meet the requirements for precise determination of particle size distribution of fine-grained soil. To address these issues, this study takes fine-grained loess from Lanzhou, Gansu Province, as the research object, and proposes a wet sieving test method suitable for fine-grained soil by modifying standard test sieves and using nylon filter cloth as a fine sieve medium. The study systematically investigates the determination of key test parameters and micromorphology. [Methods] After modifying the standard test sieves, multiple comparative tests were conducted focusing on key indicators including soil mass, water volume, test duration, and soil mass loss rate. Scanning electron microscopy (SEM) was employed to observe the micromorphology of particles in different size groups after separation. On this basis, comparative tests between the wet sieving method and the hydrometer method were conducted under conditions of full gradation and single-particle-size groups. [Results] The test results of key indicators determined the optimal test parameters as 30 g of soil and 4 000 mL of water. The duration of a single test was approximately 12 h, with the soil mass loss rate stably controlled within 3%. In contrast, the hydrometer method required approximately 48 h, indicating a fourfold improvement in efficiency. SEM results showed that the wet sieving method could achieve physical separation of fine-grained loess particles. The short-axis dimension of particles was identified as the key parameter controlling the sieving process. A high proportion of particles in the corresponding size groups after separation verified the reliability of the method. Comparative test results showed that the contents of clay and colloidal particles measured by the wet sieving method were significantly higher than those obtained by the hydrometer method, indicating that the traditional hydrometer method markedly underestimated the content of particles smaller than 0.005 mm in fine-grained loess. Further tests on single-particle-size groups confirmed that the hydrometer method exhibited large deviations in particle size determination for fine fractions, whereas the proposed method could directly and accurately reflect the particle size distribution. [Conclusion] This study indicates that the modified wet sieving method based on nylon filter cloth can accurately determine the particle size distribution of fine-grained loess and effectively compensate for the limitations of the traditional hydrometer method in fine particle testing. This method provides reliable technical support and experimental basis for precise geotechnical testing, engineering disaster prevention, and structural safety assessment in loess regions.