Our investigations showed that plants of Cuscuta japonica Choisy could parasitize those of Solidago canadensis L. in wild field. The individuals of S. canadensis parasitized by C. japonica were dwarfish, with scorch leaves, and their growth was restrained severely. The eco-physiological indices of S. canadensis after being parasitized by C. japonica were measured under the experimental conditions, and the anatomical features of the parasitism were also observed. The main results were as follows: (1) the parasitic roots of Cuscuta japonica were able to parasitize the stems and leaves of S. canadensis by penetrating the stem and leaf epidermis to the pith of S. canadensis, and therefore, absorb a large amount of water and nutrition from the host, which enhanced its biomass and stunt the host; (2) after 50-60 days being parasitized, the maximal net photosynthesis rate of S. canadensis was down to 58.81%, the light compensation point up to 153.39%, the proline up to 287.57%, and MDA content up to128.64% in comparison with the controls; but the content of dissoluble protein and sugar declined 43.45％ and 44.47％ respectively; (3) the normal individuals of S. canadensis entered into flower bud stage in the middle of September, and full flowering stage in the middle of October; in contrast, those parasitized by C. japonica produced flower buds at the end of October, and its inflorescence length was only one-tenth of the controls. Plants of S. canadensis could not produce flower buds if the biomass (dry weight) of its parasitic plant (namely, C. japonica) exceeded 13.14% of its dry weight. The results above showed that C. japonica could restrain the growth and sexual reproduction of S. canadensis effectively. Therefore, C. japonica might be a promising species for controlling the invasiveness of S. canadensis.