This paper describes a reversed flow injection spectrophotometric method for determination of manganese(II) at low levels. A new chromogenic system TCMOPPH2-Cd(II)-imidazole has been set up using a novel synthetic chromogenic reagent meso-tetra[4-(carboxymethylenoxy)phenyl]porphyrin [TCMOPPH2] by a substitution reaction of Mn(II) and Cd(II)-TCMOPP complex. The linear range extends from 0.01 to 1.75mg.L−1with a sampling frequency of 90 h−1. Co(II), Cu(II), Cr(III) and Fe(II) exert no interference until their concentrations are up to 30, 32, 15 and 23 times of that of Mn(II), respectively. The proposed method is proved to be simple, sensitive and suitable for on-line determination of Mn(II), and it has been used in analysis of Mn(II) in water samples of Liaohe Oil Field water treat plant during the on spot purification. The detection limit was 2.8μg.L−1.