Resin adhesive restorations are susceptible to oral fluid contamination and greatly influenced by dentinal tubule fluid because of pulpal pressure, especially when the restorative cavities are near gingival tissues. This study designed a novel model to evaluate the microleakage of self-adhesive flowable composite and traditional resin adhesives under simulated subgingival cavity preparations and pulpal pressure. We applied three different adhesive systems, include Vertise Flow, Optibond all-in-one, and Optibond S, on premolars with V-shaped cavity. All samples exhibited good marginal sealing at resin-enamel interfaces. At resin-dentin interfaces, microleakage in control group was similar among different adhesive systems. Microleakage in group pulpal pressure was greater than that in control group for all adhesive systems except Vertise Flow. All adhesive systems in pulpal pressure and simulated subgingival group exhibited significantly greater microleakage. In total, Vertise Flow exhibited better marginal sealing under pulpal pressure than other traditional adhesives.