Abstract: A terminology of “Eco-Economic Period Threshold For Weed Control” was proposed. On this base, field experiments were conducted during 1984, 1985 and 1991 in Beijing region to fit the functional relationships between the relative weedy or weedfree days of weeds and the relative yield of summer soybeans. Calculating models of the eco-economic period threshold for the weed control were established.Results calculated by using these models showed that the eco-economic period threshold for controlling the weeds in summer soybeans ranged from 12.4% to 31.0%, namely from 11 to 27 days after emergence in Beijing region. This is the critical period to remove all of the weed damage, to save weed control cost and to maximally utilize the role of the weed growth in reducing the loss of water, nutrients and soil during the growth season of summer soybeans.