Interfacial polymerization (IP) has become a dominant technique for preparing thin film composite (TFC) membranes. Deep insight into the IP process is crucial for the design of TFC membranes with optimal performance. According to the conventional mechanism, a PA layer would be formed upon a support membrane during the IP reaction. In order to replenish this mechanism, a phenomenon denoted as "support membrane pore blockage (SMPB)" is explored in this study. Different experiments were designed to show how SMPB affected the membrane performance by regulating the monomer concentration, the pore size of the support membrane, the lag time after aqueous phase coating, and using different IP techniques. Various conceptual models were proposed to expound SMPB. The results indicated that the penetration of polyamide polymer into the support membrane may lead to a dual effect: (i) excessive penetration can cause support membrane pore blockage, and therefore increase the flow resistance and decrease the water permeability; (ii) it can produce a "mechanical interlocking effect", which can enhance the adhesion between the PA layer and the support membrane, thus improving the TFC membrane stability. The SMPB mechanism can be adopted to elucidate phenomena related to membrane synthesis and to understand the structure-property-performance relationship.