Perennial ryegrass (Lolium perenne L.) is a cool-season forage and turf grass species cultivated world-wide. Natural variations of stress tolerance of perennial ryegrass varieties and mechanisms of stress tolerance are of great interest for breeders. In this study, stress tolerances of seventeen varieties and function of HSFC1b have been characterized. The results showed seventeen varieties exhibited contrasting tolerance to drought, heat and cold stresses. Tolerant varieties had higher proline than sensitive ones after stress treatments. Drought, heat and cold stresses extensively reprogrammed transcriptome of perennial ryegrass and modulated expression of 4797, 4329 and 4635 genes, respectively. LpHSFC1b was significantly induced after stress treatments and characterized as the core regulators through protein interaction network analysis. Ectopic expression of LpHSFC1b in Arabidopsis improved plant thermotolerance, but not drought and cold tolerances. Transgenic Arabidopsis had decreased electrolyte leakage (EL), malondialdehyde (MDA) content and chlorosis⁄necrosis severity indices, and increased expression of heat stress responsive genes. These data indicated that perennial ryegrass exhibited divergent stress tolerances, and HSFC1b likely functioned as a positive regulator in heat stress response, that may provide detailed insight for understanding of abiotic stress response clues in perennial ryegrass.