In HCl medium,trace o-dinitrobenzene can sensitively inhibit the discoloration reaction of neutral red oxidized by KIO4. The optimum neagent is 0.55 mL neutral red, 0.60 mL HCl, 0.6 mL KIO4, respectively. The neaction ttmperatune is 80℃， neaction time is 6 min. The optimum condition and kinetic parameters are investigated in detail,a new method for determination of trace o-dinitrobenzene is proposed.　The　detection limit is 7.7×10-7　g/L and the linear range is 0.0～4.0　μg/L o-dinitrobenzene.　It has been　applied to the determination of trace o-dinitrobenzene in environmental water sample with　satisfactory results.