Italian cocklebur (Xanthium italicum Moretti) invades liquorice farmlands, but specific consequences of the invasion remain unclear. Therefore, in this study, we simulated soil fertility levels of liquorice farmlands and performed replacement experiments to compare the relative competition intensity between X. italicum and Chinese liquorice (Glycyrrhiza uralensis), and investigated the differences in the effects of planting patterns (monoculture and mixture) on the growth and reproduction of the two species. Under monoculture treatment, the growth of X. italicum was more than that of G. uralensis. Vegetative and reproductive growth of X. italicum was significantly increased under mixture culture treatment compared to that under monoculture treatment; however, growth and clonal reproduction ability of G. uralensis were significantly suppressed. The biomass and contents of secondary metabolites of G. uralensis were significantly decreased under mixed culture treatment, and rhizome development was completely inhibited after mixed planting with X. italicum. The differences in vegetative growth between the two plants became larger under mixed culture treatment. Therefore, X. italicum exhibited significantly higher competitiveness than G. uralensis when coexisting within a community, and the X. italicum exerted significantly negative effects on the growth, reproduction, nodule development, yield of medicinal materials and contents of medicinal secondary metabolites of G. uralensis.