An electrochemiluminescence (ECL) sensor based on a benzo[3]uril-modified glassy carbon electrode with sensitized luminescence, with the coexistence of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as the coreactant, was successfully constructed. The sensitization mechanism was proposed by analyzing the results of the control experiments for establishing the relationship of the luminescence effect with the concentration of HEPES. Under the optimized conditions, the fabricated sensor system was applied for the detection of Fe3+ in an aqueous solution with good sensitivity and selectivity. A low detection limit of 0.41 nM was achieved, indicating superior sensor performance over the previous analytical methods. The ECL sensor system was employed for the detection of Fe3+ in human serum samples to produce excellent recoveries ranging from 96.17% to 101.81%.