The invasive plant Alternanthera philoxeroides (Amaranthaceae), which is native to South America, has spread widely throughout China. The native holoparasite Cuscuta australis (Convolvulaceae) parasitizes A. philoxeroides in the field and can significantly inhibit the growth of this invasive plant. Therefore, C. australis might be a potential biological control agent to control existing populations and/or prevent further spread of A. philoxeroides. Exogenous calcium ions can effectively alleviate the deleterious effects of stress on plants, and improve their stress resistance. Previous studies have shown that there can be an antagonistic interaction between parasitism and exogenous calcium ions. Specifically, supplying exogenous Ca²⁺ ions increased the collenchyma and cortex thickness of A. philoxeroides stems, thus improving its resistance to parasitic plants. However, no empirical studies have been conducted to verify the hypothesis that exogenous calcium ions can reduce the damage caused by parasitic plants to their hosts. In this study we conducted a greenhouse experiment to test the relationship between exogenous calcium ions and C. australis, in terms of its effect on the growth of A. philoxeroides. C.australis had significantly negative effects on the root, stem, and leaf biomass of A. philoxeroides. There were significant interactive effects between C. australis and exogenous calcium ions on the stem and leaf biomass of A. philoxeroides. There was a significant interactive effect between exogenous calcium ions at all tested concentrations (2-8 mmol/L) and C. australis on the stem biomass of A. philoxeroides, while this interactive effect was significant only between 8 mmol/L Ca²⁺ and C. australis for leaf biomass of A. philoxeroides. Exogenous calcium ions at all tested concentrations significantly decreased the deleterious effects of parasitic C. australis on the stem biomass of A. philoxeroides, while only 8 mmol/L Ca²⁺ significantly decreased the deleterious effects of C. australis on the root biomass and leaf biomass of A. philoxeroides. These results confirmed that there is an antagonistic interaction between exogenous Ca²⁺ and parasitism of C. australis. That is, exogenous Ca²⁺ could alleviate the deleterious effects of parasitic C. australis on its host, A. philoxeroides. When supplied at concentrations of 2 to 6 mmol/L, Ca²⁺ did not significantly affect the malondialdehyde content in the leaves of A.philoxeroides parasitized by C. australis, but peroxidase activity was significantly decreased. These findings suggest that changes in cellular biochemistry are associated with the ability of Ca²⁺ to alleviate the deleterious effects of C. australis on A.philoxeroides.