Salvia miltiorrhiza-derived carbon dots (SmCDs) have a potent antioxidant capacity. Squamosa promoter-binding protein-like (SPL) transcription factors respond to both heat and cold stress. However, the molecular mechanisms underlying the regulation of both high temperature (HT) and low temperature (LT) stress by SmCDs through SPL-mediated ascorbate (AsA) biosynthesis and recycling pathways remain unexplored. Therefore, we systematically identified 29 BrSPLs and 16 genes related to AsA biosynthesis and recycling pathways in flowering Chinese cabbage. BrSPL11.1 and three genes (BrGalDH, BrAPX, and BrDHAR1) involved in AsA biosynthesis and recycling pathways were induced by both HT and LT, and their expression patterns were modulated by SmCDs. Functional analyses revealed that the expression of BrGalDH, BrAPX, and BrDHAR1 was suppressed in BrSPL11.1 overexpressing (BrSPL11.1-OE) plants, leading to reduced AsA content and reactive oxygen species (ROS) accumulation, as well as enhanced sensitivity to thermal extremes. Conversely, atspl11.1 showed inverse phenotypic and biochemical trends. BrSPL11.1 directly bound to the promoters of BrGalDH, BrAPX, or BrDHAR1 and suppressed their transcription. Silencing BrGalDH, BrAPX, or BrDHAR1 decreased the AsA content and increased ROS accumulation, decreasing HT and LT resistance. SmCDs application effectively elevated the AsA content and attenuated ROS accumulation, alleviating oxidative damage under both stress conditions. Our results established that BrSPL11.1 acts as a dual-temperature stress repressor and that SmCDs enhance HT and LT resistance by regulating BrSPL11.1-mediated AsA biosynthesis and recycling pathways.