Abstract: Annual precipitation in the tableland of the northern Qian County during the years of 1987-2000 can be divided into three types：plenty of rainfall，normal and dry However，drought and rainfall deficiency are the main characters during this period of time The restoring period of soil moisture is in summer fallow time，while its consumption and losing period is during the growth of wheat and the soil moisture in 0～2m soil layer declines to the minimum of the whole year in harvest time Wheat yield has a direct correlation with moisture consum ption in the field and water use efficiency，while it has a negative correlation with water use coefficient．