Abstract: Barnacles, as common fouling organisms in mangrove forests, seriously harm the growth of mangrove plants. The massive attachment of barnacles often leads to the death of mangrove seedlings in newly planted forests. The Scylla paramamosain and barnacles share a predator-prey relationship, with the crab serving as a significant predator of barnacles. We aim to investigate the predatory behavior and predation efficiency of the Scylla paramamosain on barnacles. We employed Scylla paramamosain of four body mass classes (100 g, 140 g, 180 g, and 260 g) in predation experiments where they were exposed to barnacles. We conducted predation experiments using Scylla paramamosain to target barnacles (Euraphia withersi and Amphibalanus amphitrite) attached to twigs of the mangrove plant Avicennia marina. Through video surveillance, we observed that Scylla paramamosain detach barnacles from twigs using their chelae, then transfer them to their mouthparts to crush the shells and consume the soft tissue within. The results indicate that both the body mass of Scylla paramamosain and the size of barnacles exert highly significant influences on the crabs' predation efficiency (p<0.01). All body mass classes of Scylla paramamosain exhibited a predation preference for larger barnacles (mean shell length: 6.81 mm). By measuring prey consumption as an indicator of predation efficiency (y), we observed that Scylla paramamosain reached their maximum predation efficiency on barnacles at Day 1 of the experiment. Moreover, the relationship between predation efficiency (y) and Scylla paramamosain body mass (x) can be modeled by the regression equation: y=-112.790+2.146x-0.006x² (R²=0.868, p<0.01). The regression equation reveals that the predation efficiency of Scylla paramamosain on barnacles initially increases but subsequently declines as crab body mass rises, exhibiting a distinct unimodal trend. On Day 3 of the experiment, the 180 g body mass class demonstrated the highest cumulative predation efficiency (94.4%), while the 260 g group showed the lowest efficiency (35.4%). Field experiments likewise confirmed the effective biocontrol potential of Scylla paramamosain against barnacle colonization on mangrove seedlings, with over 70% of seedlings achieving barnacle clearance rates exceeding 50%.