Abstract: Available precipitation is a very important concept for the complement of soil moisture and crop growth. Generally rainfall greater than 5 mm is considered as available precipitation. There are many factors influencing available precipitation. In different regions and weather conditions, the minimum available precipitation will be different. From the point of view of soil moisture change and according to the definition of available precipitation, a preliminary study of the minimum available precipitation in Yuzhong was made, with the data measured during June 2006 to March 2011 provided by the Semi-Arid Climate Observatory and Laboratory. By analyzing the minimum available precipitation of different soil depths in the different seasons, under different temperature and vegetation conditions, we found that the minimum available precipitation of 5 cm, 10 cm, 20 cm soil layers are 4 mm, 5 mm, 8 mm. As for the seasonal distribution, the minimum available precipitations in summer are the highest of all soil layers. In spring and autumn the values are closer to each other. In the warmer year the minimum available precipitation is higher than that of the low temperature year; in the growing season it is higher than the value of the non-growing season. With the increase of the amount of precipitation, soil moisture increment exponentially increases when precipitation is larger than the minimum available precipitation and of small magnitude. Then precipitation conversion is higher. And when precipitation reaches a certain level, at which the rainfall intensity is larger than the soil infiltration rate, the water loses in the form of runoff, the rate of the soil moisture increment reduces, and the conversion rate of the precipitation tends to a constant value. The conversion rate of precipitation in 0-20 cm soil layer reaches 70%.