Abstract: The relation of the soil water content, the soil temperature and their interaction to urea transforming into ammonium with 4 grades of soil temperature (15, 20, 25 and 30℃) and 5 grades of the soil water content (40, 60, 80, 100 and 120g/kg) was studied in the room. The urea transformation was analyzed by determining the content of ammonium nitrogen in the different time with FIA. There were two aims for the study, the first was to make it clear that the dynamic process of urea transforming into ammonium affected by the interaction of soil water content and soil temperature and the second was to improve the utilization efficiency of urea in arid farmland. The results showed that the transformation efficiency of urea into ammonium was promoted, when the water content of soil or the temperature of soil increased. There was a highest content of ammonium in each treatment, which appeared on about the firth day to the tenth day. The highest content of ammonium increased when the soil water content and the soil temperature rose. The dynamic model that urea transformed into ammonium in soil could be expressed by the eolation as Y = a + bt + c/t (Y is the content of ammonium, t is time, a, b and c are coefficients) . The three coefficients all related to the soil temperature and water content. When the soil temperature unchanged, a and c increased with the soil water content, but b decreased. When the soil water content unchanged, a, b and c increased with the soil temperature. The interaction of the soil water content and the soil temperature was existed under this trial conditions. The effect of the interaction between the soil water content and the soil temperature on the a value was negative, and on the b and c values were positive. There were the optimum temperature and soil water content range for urea transformation into ammonium, respectively under the trial condition. The optimum temperature range was between 20 and 25℃, and optimum soil water content was 80g/kg.