Nitrification inhibitors exert inhibition function in soil nitrification process (NH+4-N to NO-3-N) and are widely applied in order to improve N fertilizer use efficiency. Before the new nitrification inhibitor is used, its effects on denitrification process must be investigated and denitrifying enzyme activity (DEA) is an effective indicator to show this process. In the present paper, a mass spectrometery(MS) method was taken to measure the denitrifying enzyme activity in the new nitrification inhibitor 3, 4-dimethylpyrazole phosphate(DMPP) incubation system. The results showed that the method measured the concentration of N2O in the incubation system accurately and the concentration measured by MS had marked correlation with that measured by gas chromatogram(GC) (MSN2O=-0.45+1.03GCN2O, R2=0.995). At the same time, enrichment of 15N2O and 15N2 was measured to discriminate the nitrate reductase and N2O reductase. Differed with traditional method, the enzymatic inhibitor-acetylene was not appended. The results showed that DMPP had no influence on the denitrifying enzyme activity and indicated that the denitrification process was not influenced by DMPP.