This research investigates the mesoscopic damage mechanisms in hydroxyl-terminated polybutadiene （HTPB） propellants under thermal-mechanical coupling. Experimental characterization and theoretical analysis were employed to analyze these mechanisms at different environmental temperatures （50， 70， and 90 ℃） and loading cycles. At 50 ℃， the propellants underwent approximately 3000 and 10800 loading cycles； at 70 ℃， 1800， 3600， and 7030 cycles； and at 90 ℃， around 1800 cycles. The findings indicate that the mesoscopic damage in HTPB propellants， exacerbated by thermomechanical coupling， is more pronounced than that caused by a single aging factor. This damage primarily results from two processes： thermal degradation of the matrix， diminishing both its load-bearing capacity and adhesion to particles， leading to particle dewetting； and the subsequent exacerbation of the matrix"s thermal degradation by this dewetting. The above interaction makes the mesoscopic damage more severe. Moreover， the damage intensifies with increasing aging temperature， but extremely high temperatures modify the mesoscopic damage mechanism. Furthermore， the study emphasizes the importance of selecting an appropriate number of loading cycles to assess significant mesoscopic damage in HTPB propellants. Notably， substantial damage occurs when the loading cycle ratio （） at 50 ℃ and 70 ℃ exceeds 0.281 and 0.330， respectively.