Abstract: Free swelling rate is a critical parameter for identifying and classifying the swelling properties of expansive soils. In the current " Standard for Geotechnical Testing Method GB/T 50123-2019", numerous influencing factors affect the free swelling rate test, and the testing mechanism remains incomplete. It is crucial to refine the free swelling rate test mechanism to enhance the parameter's scientific validity. This study examines artificially prepared expansive soils, thoroughly investigating the two main free swelling rate testing methods: China's GB/T 50123-2019 and the American ASTM D5890-19. The study discusses and analyzes the influencing factors and relationships between these methods, proposing a correction method for the free swelling rate based on mass-to-volume ratio. The results indicate that factors such as soil type, particle size, and operational procedures significantly impact the free swelling rate (δ_ef/GB/T 50123-2019) and the swell index (SI/ASTM D5890-19). For the same soil sample, discrepancies in free swelling rates are primarily caused by variations in particle size, shape-induced mass changes, and stirring frequency. Swell index changes are influenced by the degree of soil film coverage. As the proportion of sodium bentonite in the prepared expansive soil increases, the swelling volume, free swelling rate (δ_ef), and swell index (SI) increase linearly, with the growth rates of the free swelling rate and swelling volume significantly affected by particle size and shape. With an increase in sodium bentonite proportion, the specific swelling volume of the soil exhibits a linear function increase under GB/T 50123, while it follows a downward-opening power function under ASTM D5890. To offset mass differences resulting from particle size, the study proposes using specific gravity Gs to correct the free swelling parameters of the soil. The corrected true free swelling volume Vtsf is not affected by particle shape, particle size, compaction, or other mass-related factors and exhibits greater physical rigor, effectively reflecting the inherent swelling properties of the soil.