Tris(2,4,6-trimethoxyphenyl)polysulfonemethylene quaternary phosphonium chloride (TPQP-Cl) was blended with polysulfone (PSf) in different compositions to fabricate PSf/TPQP-Cl composite ultrafiltration membranes using the nonsolvent-induced phase separation method. The blending of polymers was confirmed by attenuated total reflectance infrared (ATR-IR) spectroscopy. Surface and cross-sectional morphologies of membranes were characterized using scanning electron microscopy (SEM). The SEM images showed that the PSf/TPQP-Cl membranes had a typical asymmetric structure. The X-ray photoelectron spectroscopy (XPS) and contact angle analysis revealed the enrichment of TPQP-Cl in the supporting layer of the membrane. In addition, water content, porosity, contact angle, pure water flux, and molecular weight cutoff were measured to study the influence of addition of TPQP-Cl. In particular, the addition of TPQP-Cl led to greatly increased water flux without significantly increased molecular weight cutoff; the PSf/TPQP-Cl membranes exhibited up to 7.3 times higher water flux than the pure PSf membrane at similar rejection properties. This work provides an effective way to tailor ultrafiltration membrane structure to achieve high flux while maintaining rejection properties.