A facilely catalytic approach to prepare weakly entangled ultra-high-molecular-weight polyethylene (UHMWPE) with a fine particle size is reported with the polyhedral oligomeric silsesquioxane (POSS)-modified MgCl2-based Ziegler-Natta catalyst. CO in situ FTIR spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and N2 adsorption and desorption experiments revealed that the POSSs can coordinate with the MgCl2(110) plane, acting as adhesives with a cage structure to be in situ embedded into the MgCl2-based support. This in situ embedding of POSS facilitated the uniform dispersion of POSS nanoparticles, contributing the isolation effect for active TiCl4 sites in the molecular scale, thus synthesizing the weakly entangled UHMWPE with exceptional activities. Simultaneously, the MgCl2-based catalysts afforded UHMWPE particles with small sizes, where POSSmodified catalysts brought the UHMWPE particle size down to below 200 μm, thanks to their considerable activities. To the best of our knowledge, this is the first time that nascent UHMWPE with a less entangled state and a fine particle size was synthesized. This combination of structural advantages enables substantial improvements in the toughness/stiffness/strength balance of nascent UHMWPE, especially for the impact strength which can reach 113.6 kJ/m2 with a relatively low molecular weight