Detection of backscattering signals (BSS) generally suffers from the interference of reflected light, and it is hard to apply these signals for analytical purpose. Herein we provided an optical assembly, which effectively eliminated the interference of reflected light so that the scattering signals of analyte could be measured distinctly. With this assembly, chlorine in human urine could be detected with the limit of detection (LOD) of 2.0 ng/mL by measuring the enhanced BSS signals produced between the interactions of chlorine with silver nitrate.