Abstract: The ecoeconomic threshold for controlling weeds in the field of summer maize in Beijing and Yantai regions has been investigated.The mathematic relationship between the loss rate of maize yield and weed density is established.The results show that,with the increase of weed density,the competition between weeds and maize becomes intense,and maize yield decreases slowly.When weed density is 200 plants per square meter,the loss rate of maize yield is 53.07 percent.The results also show that the ecoeconomic threshold for weed-controlling in the field of summer maize in the North China is 4.21～6.91 plant/m²，namely,weeds could grow freely when they are under the threshold and the influence on maize yield is quite little.However,as to weeds which are over this threshold,timely and effective measures for controlling weeds should be necessary.