Abstract: Polygonum lapathifolium L. is a troublesome weed seriously damaging soybean growth and reducing soybean yield in soybean fields. To determine economic infestation level and economic threshold of P. lapathifolium in soybean fields,a field experiment was carried out to study the effects of P. lapathifolium of different densities (0, 10, 20, 30 and 40 plant/m2 ) on the soybean growth and yield by the method of additive series experiment. [Result] Results showed that increasing density of P. lapathifolium significantly reduced soybean’s pods per plant and yield. There was no significant impact on soybean plant height.The linear function model was the best in describing the relationship between P. lapathifolium density and soybean’s pods per plant as y = -0.443x 39.453(R2=0.962;F=75.887;P=0.003); While the quadratic regression model as y=3027.810x2-54.395x 0.702(R2=0.972;F=34.843;P=0.028) was better for describing the relationship between P. lapathifolium as y=-23.458lnx 16.131(R2=0.9112;F=20.532;P=0.045) density and soybean yield; while the logarithmic regression model was better for describing the relationship between P. lapathifolium density and yield loss. Economic threshold of P. lapathifolium control was 1.42 plants /m2 for manual weeding , while for the chemical herbicides of 75% thifensulfuron WG, 80% flumetsulam WG,95% s-metolachlor EC were 1.88,1.82 and1.77 plants /m2, respectively. S[Conclusion] Herbicide application effectively and economically controlled P. lapathifolium in soybean fields.