Chain entanglement was very important for adjusting the processability and mechanical property of nascent ultrahigh molecular weight polyethylene (UHMWPE). So far, it is still a mystery to unravel the formation mechanism of entanglements when the ethylene polymerization is conducted by the heterogeneous catalysts. In this study, a series of weakly entangled UHMWPE was synthesized by the polyhedral oligomeric silsesquioxane/MgCl2 nanoaggregates modified Ziegler-Natta catalysts. The structure of nanoaggregates was evaluated by X-ray photoelectron spectroscopy, density functional theory simulations, and scanning probe microscope experiments, where the coordination strategy of MgCl2 and hydroxyl of POSS was investigated. These nanoaggregates presented extremely low activity on ethylene polymerization and were proved to serve as isolators for separating the active sites and growing chains. The entanglement density of nascent UHMWPE (reflected by the value of initial storage modulus G′(t=0)) was exponentially decayed with the polymerization activity of increased numbers of nanoaggregates. Importantly, this exponentially decayed effect contributed by increased numbers of POSS/MgCl2 isolators offset the rapid power function increment of entanglements upon rising the temperature, which was the essential reason for the successful synthesis of weakly entangled UHMWPE even at 85 °C. Finally, we have proposed the dependence and sensitivity of G′(t=0) (i.e., indicating the initial entanglement density of nascent polymers) on the polymerization activity, which was able to trace the formation of entanglements during polymerization through the POSS modified heterogeneous catalyst.