Surface roughness, a conventional index widely applied in wind erosion protection, plays a crucial role in evaluating the effectiveness of engineering measures such as forest and sand barriers. Despite its prevalent use, practical applications reveal its limited accuracy. Consequently, we introduce a novel index termed “boundary layer displacement thickness”. Through wind tunnel simulations, we assessed the wind speed frequency distribution of various sandbag barriers. We computed the mean wind speed, wind prevention efficiency, and also scrutinized the wind speed flow field to validate the precision of both roughness and boundary layer displacement thickness measurements. The findings indicate that an increase in the length of the polylactic acid sandbag barrier corresponds to an upward trend in average wind speed. Specifically, the average wind speed within the 1.5-m and 2.0-m barriers is 1.13 and 1.24 times that of the 1.0-m barrier, respectively. Concurrently, wind prevention efficiency experiences a decline, with values exceeding 0.6 for the 1.0-m barrier, 0.5 for the 1.5-m barrier, and 0.4 for the 2.0-m barrier. Analysis of the wind velocity field data reveal that the high-speed zone expands as the barrier grid size increases, signifying a weakened protective effect with large sand barriers. Notably, the boundary layer displacement thickness demonstrates a consistent decreasing trend with increasing barrier size, aligning with the aforementioned pattern. However, surface roughness exhibits an anomalous pattern, initially decreasing and then increasing. In summary, the boundary layer displacement thickness proved to be more accurate and emerged as a promising supplementary reference indicator for evaluating the efficacy of wind erosion protection measures.