Based on the 30-year meteorological data, 20-year crop planting area data and coefficient of irrigation water utilization factor of 11 weather stations in Baojixia Irrigation district, the water demand of agriculture was calculated by Penman formula and quota method respectively, then trend analysis and gray relational analysis were used to analyze its changing characteristics and driving forces and discuss the results of the calculation and actual agricultural water consumption. The consequence suggests that under the combined action of many kinds of environmental factors, the agricultural water demand in Baojixia irrigation district shows a decreasing trend during 1991-2010. The main driving factors affecting the change of agricultural water demand are the reduction of planting area and the reduction of planting ratio of grain crops. The actual water consumption in middle years and in the arid years is close to the agricultural water demand based on the quota method in the case of under-sufficient irrigation. The actual water consumption in wet years is larger than the agricultural water demand calculated based on the quota method, less than the agriculture water demand calculated based on the Penman formula. Based on the above conclusions and considering the actual water demand in irrigation districts, a reasonable method for determining agricultural water demand is proposed.