It studied the relationship between the dam and reservoir control area ratio to the total area,the dam and reservoir capacity unit area(DRCA),the critical DRCA and sediment reduction effect in the northwest of Shanxi and north of Shaanxi in the Hekouzhen-Longmen area of middle reaches of the Yellow River.The results show that:(1) In the northwest of Shanxi,without the dam and reservoir engineering,the control measure of the slopeland accounted for 14% in sediment reduction.The sediment reduction effect increases with the increase of the dam and reservoir control area,reservoir capacity and DRCA: the increase of 100 km2 in dam and reservoir control area can increase 2% in sediment reduction effect;the increase of 4 000 000 m3 in the reservoir capacity can increase 1%;and the increase of 10 000 m3/km2 in the DRCA can increase 10%.(2)In the north of Shaanxi,the sediment reduction is mainly attributed to the dam and reservoir engineering,the DRCA of 50 000 m3/km2 can increase 10% in trap efficiency;the DRCA should be more than 60 000 m3/km2 if the sediment reduction effect of the comprehensive watershed control of soil and water conservation achieves 20%.(3)In the areas with high and coarse sediment yield of middle reaches of Yellow River,if the sediment reduction effect reaches 40%,the DRCA should excess 160 000 m3/km2.