One of the main factors in the successful invasion of invasive plants is their allelopathy on the growth performance (especially seed germination and seedling growth) of neighboring plants. Salt stress, mainly mediated by soil salinization, may affect or even facilitate the process of invasion of invasive plants via their allelopathy. The aim of this study was to evaluate the allelopathy effect of four Asteraceae invasive plants, including Canada goldenrod (Solidago canadensis L.), horseweed (Conyza canadensis (L.) Cronq.), rallway beggaricks (Bidens pilosa L.), and daisy fleabane (Erigeron annuus (L.) Pers.), on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under a gradient of salt stress in an hydroponic incubation experiment. Salt stress significantly reduced seed germination and seedling growth of L. sativa. These four invasive plants are known to negatively affect seed germination and seedling growth of L. sativa through their allelopathy. The allelopathy of S. canadensis was stronger than that of the three other invasive plants. Salt stress significantly intensified the allelopathy of the four studied invasive plants (especially S. canadensis), and the facilitation of salt stress on the allelopathy of the four invasive plants (especially S. canadensis) significantly increased with increasing intensity of salt stress. Therefore, the increased level of salt stress may facilitate the process of invasion of the four studied invasive plants (especially S. canadensis) via their increased allelopathy negatively affecting the seed germination and seedling growth of neighboring plant species.