Abstract: Barnyardgrass (Echinochloa crusgalli) is an important weed in double-cropped paddy fields in southern China and a significant threat to rice production. To determine the influence of barnyardgrass on the growth of double-cropped paddy rice and its economic infestation threshold, the growth and yield of paddy rice under different barnyardgrass densities were investigated using field experiments and modelling. Results showed that plant height, tiller numbers, effective spikes, grain numbers/panicle, 1000-grain weight and grain yield of double-cropped early and late rice decreased with increasing barnyardgrass density. The exponential regression model “y=be^ax” was able to adequately describe the relationship between barnyardgrass density and tillers, effective spikes and yield of double-cropped early rice, while a quadratic regression model “y=ax²+bx+c” was best for plant height, grains/panicle, 1000-grain weight and yield loss. A quadratic regression model “y=ax²+bx+c” was best able to describe the relationship between barnyardgrass density and plant height, tillers, effective spikes, grains/panicle, 1000-grain weight, grain yield and yield loss of double-cropping late rice. Herbicides (butachlor, quinclorac, or penoxsulam) were applied to control barnyardgrass; the economic infestation levels for barnyardgrass were 1.64%-2.91% and 1.28%-2.28% in double-cropped early and late paddy fields, respectively, and the economic thresholds were 0.63-1.23 plant/m² and 1.30-1.85 plant/m², respectively. Barnyardgrass severely inhibited rice growth and yield. Herbicide application effectively and economically controlled barnyardgrass.