Abstract: In the enrichment system of marine nitrite-oxidizing bacteria with 100mg/L of initial concentration of NaNO2,in order to qualitatively detect the nitrate produced by nitrite-oxidizing bacteria,the residual nitrite must be removed firstly, which can be made successfully by adding 20μL of 1.0 mol/L HCl and 10 - 20μL of 50g/L NH4SO3NH2 to per milliliter of media. Then, the nitrate can be qualitatively detected by using diphenylamine reagent. The minimal concentration of NaNO3 that can be detected is about 20mg/L. In the different enrichment system with different initial concentration of NaNO2, the dose of HCl and NH4SO3NH2 used to remove NaNO2 can be regulated in proportion as the initial concentration of NaNO2, but the maximum of its initial concentration should not surpass 200mg/L. Also, this kind of qualitative detecting method for nitrate can be applied in the initial enrichment system of freshwater nitrite-oxidizing bacteria.