An incubation experiment was conducted to study the effects of different amounts of humic acid(0,5%,10%,25%,50%,75%HA)on soil nitrogen transformation and ammonia volatilization losses.The results showed that compared with control,1)humic acid reduced ammonia volatilization significantly by an average of 12.08%,meanwhile,the more humic acid added,the greater inhibitory effect showed on ammonia volatilization.2)In the prophase of cultivation,the 5%~50% humic acid improved the soil urease activity and the average increase was 35.13% at the fifth day,but 75% humic acid decreased soil urease activity by 13.23%,then enhanced it in the later period of incubation(14ndday). 3)Humic acid increased soil ammonium nitrogen content，the increases were enhanced with the increase of humic acid adding proportions.The content of ammonium nitrate treated with humic acid increased averagely by 39.63% at the 112nd day.4)During the whole culture period,the soil apparent nitration of humic acid treatments decreased by 17.20% averagely,and the higher the amount of humic acid,the lower the soil apparent nitration rate.These results have important significance to reveal the role of humic acid in the regulation of soil nitrogen,reduce nitrogen loss and increase the utilization efficiency of nitrogen fertilizer.