The allelopathy exerted by invasive plants is a crucial factor in their successful invasion. The allelochemicals of invasive plants are predominantly produced by their aboveground (mainly leaves) and belowground (i.e., root) parts. Furthermore, drought is becoming increasingly prevalent, which may result in an intensification of the allelopathy of invasive plants. However, there has been a paucity of research investigating the effects of drought stress on the allelopathy of invasive plants, particularly with regard to the aboveground and belowground parts. This study aimed to elucidate the allelopathy of the independent and combined aboveground and belowground parts of the notorious Asteraceae invasive plant Solidago canadensis L. on the seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under drought stress. Solidago canadensis (particularly aboveground part, and combined aboveground and belowground parts) on L. sativa seed germination was clearly evident, yet it also resulted in an increase in L. sativa seedling growth. The aboveground part of S. canadensis exhibited a more pronounced allelopathy on L. sativa seed germination than its belowground part. The belowground part of S. canadensis exerted a more pronounced allelopathy on L. sativa seedling growth than its aboveground part. The combined aboveground and belowground parts of S. canadensis exhibited a synergistic effect on L. sativa seed germination, while exhibited an antagonistic effect on L. sativa seedling growth. Drought stress significantly intensified the allelopathy of S. canadensis. Severe drought stress was found to intensify the allelopathy of the belowground part to a greater extent than that of the aboveground part. Accordingly, drought stress, especially severe drought stress, may facilitate the invasion process of S. canadensis via the intensified allelopathy, especially exerted by the belowground part.