Invasive alien species (IAS) can distinctly inhibit seed germination and seedling growth performance (SGeSGrP) of indigenous species via the allelopathy. The progressively increased drought stress can potentially affect the allelopathy of IAS. Thus, it is significant to illustrate the allelopathy of IAS on SGeSGrP of indigenous species under drought stress to obtain a deeper elucidation for the main driving mechanism attributed to the successful invasion. This study attempts to identify the allelopathy of IAS Solidago canadensis L. (goldenrod; using leaf extracts) on SGeSGrP of the indigenous species Lactuca sativa L. (lettuce) under drought stress [mimicked by Polyethylene glycol-6000 (PEG 6000)]. Goldenrod leaf extracts (low concentration) obviously reduced seed germination performance of lettuce but goldenrod leaf extracts (high concentration) notably reduced seedling growth performance of lettuce. Goldenrod leaf extracts (low concentration) also interestingly awarded an optimistic effect on the competitive ability for sunlight acquisition as well as water and inorganic salt acquisition, plant growing ability, and plant water content of lettuce. PEG 6000 dramatically inhibited the competitive ability for sunlight acquisition as well as water and inorganic salt acquisition of lettuce. PEG 6000 also noticeably enhanced the allelopathy of goldenrod leaf extracts on the germination speed and vitality, competitive ability for sunlight acquisition as well as water and inorganic salt acquisition, leaf photosynthetic area, and plant growing ability of lettuce. Thus, drought stress may be advantageous to the advance of goldenrod invasion process mainly via the oppressed SGeSGrP of indigenous species mediated by the enhanced allelopathy.