The increasing fishing effort of the crab pot fishery in the East China Sea has resulted in considerable pressure on swimming crab Portunus trituberculatus resources. Thus, there is an urgent need to implement conservation measures to release juvenile crabs from fishing pots to increase recruitment. In this study, sea trials were carried out to assess the effect of increasing mesh size on fishing efficiency and analyze the mesh size selectivity of crab pots for P. trituberculatus through a comparison experiment. Additionally, the effect of soak time and available population on the selective properties of mesh size was discussed based on the mixed effect model. The results showed that there were significant differences in P. trituberculatus carapace width (CW) distribution caught between the control pot with 32.0 mm mesh size and experimental pots with 52.3 mm and 59.7 mm mesh sizes. No significant difference in relative fishing power between control and experimental pots was detected according to the model fit of the catch frequency in the SELECT analysis model. The average CWs of P. trituberculatus with 50% probability of being retained (CW₅₀) were 86.9 mm for the pot with 52.3 mm mesh size and 90.9 mm for pot with 59.7 mm mesh size, whereas the selection ranges (SR) of CW were 15.9 mm and 9.2 mm, respectively. The finding that the P. trituberculatus experimental pot CW₅₀ was considerably smaller than the minimum landing size set by Zhejiang Province fishery regulations indicated that it may be impractical to effectively release undersized crabs by simply relying on increased mesh size in crab pot fisheries. The results of the mixed effect model showed that the influence of soak time and available P. trituberculatus population on size selectivity of the pots was not significant in this study.