The effect of sodium benzoate on purple soil nitrite reductase by improved α naphthylamine paminobenzene sulfonic acid colorimetric method was studied.The results showed that inhibiting ability of sodium benzoate on purple soil nitrite reductase enhanced by sodium benzoate concentration increased.V0,Km,Vmax reduced by adding sodium benzoate.It showed the sodium benzoate had uncompetitive inhibition on soil nitrite reductase.In conclusion，sodium benzoate applied in fertilization as excellent denitrification inhibitor should have good application prospect.