Interspecific interactions at different trophic levels play important roles in the structure and dynamics of communities. Studying such multitrophic interactions at the individual level could help to better model complicated interactions across trophic levels at the community level. Parasitic plants are one type of consumer that interacts with other species at different trophic levels in natural communities. Bidens pilosa L. (Compositae), an annual forb native to South America, has invaded subtropical south-western China, where it is now common. Cuscuta australis (Convolvulaceae), a holoparasitic plant, has been identified as a potential biological control agent of such invasive plants. Here, we conducted a common pot experiment to investigate how fertilization of host plants indirectly affected the growth of parasitic plants and the possible mechanisms related to the growth, biomass allocation patterns, and light-resource capture abilities of the hosts. The number of coils around the stems of the host plants, the number of haustoria, and the relative cover of C. australis on fertilized B. pilosa were higher than on non-fertilized B. pilosa. Fertilization significantly increased the vegetative biomass, reproductive biomass, and total biomass of C. australis, but had no significant effect on the reproductive mass ratio. The biomass of leaves, stems, roots, total biomass, leaf mass ratio, specific leaf area, and relative chlorophyll content of the fertilized host were significantly higher than those of non-fertilized hosts, but the root/shoot ratio and root mass ratio were lower than those of the non-fertilized host. The vegetative biomass, reproductive biomass, and total biomass of C. australis were significantly positively correlated with the biomass of the roots, stems, leaves, total biomass, leaf mass ratio, relative chlorophyll content, and specific leaf area of the host plant and significantly negatively correlated with the root/shoot ratio and root mass ratio. Our results suggested that fertilization could improve the light-resource usage and allocation of biomass to photosynthetic organs of invasive plants, which could result in rapid growth of these invasive hosts (producer) and thus indirectly promote the growth of parasitic plants (primary consumer).