Abstract: The weeds in rice fields are an important factor affecting rice production. It is important for agricultural development to emphasize both the economic and ecological benefits of rice field weed control. The scientific control of weeds in rice fields according to an eco-economic threshold is an effective way to ensure rice production and protect the environment. Leptochloa chinensis and Cyperus difformis are malignant weeds in rice fields and a significant threat to rice production. To determine the effects and eco-economic thresholds of L. chinensis and C. difformis on the yield of direct-seeded rice, field experiments were performed with 13 mixed densities and 6 coexistence periods with rice of L. chinensis and C. difformis. The results showed that the rice yield traits, such as the number of effective spikes, filled grains per panicle, 1000-grain weight, and rice yield, decreased with the increasing density and coexistence period with rice of L. chinensis and C. difformis. The rice yield loss rate was positively correlated with the mixed density and coexistence period of the two weeds, and there were significant differences among the treatments with different mixed weed densities and coexistence periods. When the density of L. chinensis + C. difformis was 8 + 8 plants per m², the rice yield was 2 236.37 kg·hm^-2 and the rice yield loss rate was 71.14%. At the average density of 4.67 plants per m² for L. chinensis and 3.50 plants per m²for C. difformis, when the coexistence period corresponded with the whole rice growth period, the rice yield was 5 138.33 kg·hm^-2 and the yield loss rate was 33.37%. Weeds at a low density or with a short coexistence period with rice had no significant effect on the rice yield. When L. chinensis and C. difformis coexisted with rice during the whole growth period, the threshold for the composit density of the two weeds was 4.14 plants per m². In addition, under the density conditions of the experiment, when the two weeds grew in a rice field for 16.7 days, the weeding should be conducted. In this study, the idea of controlling the weeds when the damage of L. chinensis and C. difformis reached the eco-economic threshold changed the traditional concept of weed control, and was conducive to reducing the applying times of herbicides, thus decreasing the herbicide application rate.