Abstract: A culture experiment was carried out to investigate effect of different fertilizer-soil ratios on NO3--N concentration and its mechanisms. The results indicated that fertilizer∶ soil =1∶400, w/w (treatment G) might be a sensitively fertilizer-soil ratio to change soil properties. The NO3--N concentration, nitrobacteria number and urease activity increased with increasing fertilizer-soil ratios which were less than G, but they decreased with increasing fertilizer-soil ratios which were higher than G. Change of soil pH resulting from increasing fertilizer-soil ratios was the main reason for changes of nitrobacteria and denitrobacteria numbers and urease activity, and was also important factor in affecting NO3--N concentration in soil.