A new postchemiluminescence (PCL) reaction was observed when urea was injected into the reaction mixture after the CL reaction of N-bromosuccinimide and dichlorofluorescein. A possible reaction mechanism was proposed based on the studies of the CL kinetic characteristics, CL spectra, fluorescence spectra, and other experiments. A new flow injection CL method for the determination of urea was established based on the PCL reaction. The relative standard deviation for the determination of urea was 1.3% (n = 11, c = 5.0 × 10−8 g/mL). The CL intensity responded linearly to the concentration of urea in the range 2.0 × 10−9–1.0 × 10−6 g/mL (r = 0.9992). The detection limit was 7 × 10−10 g/mL. The method had been applied to the determination of urea in milk with satisfactory results.