This study reveals the preparation of shear thickening fluid (STF) impregnate ultra-high molecular weight polyethylene (UHMWPE) fiber flexible composites with different areal densities of UHMWPE fabric. STF was prepared using SiO2 nanoparticles and polyethylene glycol solution (PEG). The drop weight impact test was carried out to investigate the damage effect and energy absorption mechanism of STF/UHMWPE flexible composites under low-velocity impact with various incident velocities. The results manifested that STF/UHMWPE flexible composites exhibited similar softness compared with pure fabric, but the impact resistance was significantly higher than that of pure fabric. The impact resistance was at least 4.4 times higher than that of pure fabric. The composites with various parameters had a little fracture. The fabric deformation was the main energy absorption mechanism, and the friction between impact head and nanoparticles dissipated a small amount of impact energy.