Abstract: Tengchong, located on the Yunnan–Guizhou Plateau, is a typical mountain city characterized by complex terrain. Its PBL (planetary boundary layer) and cross-border transport of pollutants, influenced by its unique geographical location, urgently require attention. Therefore, the long-term structure of the PBL and the changing characteristics of meteorological conditions in Tengchong should be studied. In this study, L-band radiosonde data of Tengchong and the atmospheric PBLH (PBL height) calculated using the Richardson number method are used to verify the ERA5 reanalysis data. Based on these data, the temporal characteristics and influencing factors of PBLH in Tengchong during 1980–2023 are analyzed using statistical methods. The results indicate that the PBLH of ERA5 correlates well with the observed PBLH in Tengchong, although the average PBLH of ERA5 is 3.7 m and 279.4 m lower than the observed PBLH at 0800 BJT (Beijing time) and 2000 BJT, respectively. The PBLH in Tengchong exhibits significant fluctuations, with upward trends in summer and autumn (0.7 m a⁻¹ and 0.2 m a⁻¹, respectively), downward trends in spring and winter (−0.1 m a⁻¹ and −0.2 m a⁻¹, respectively), and upward trends during the dry and wet seasons (0.05 m a⁻¹ and 0.3 m a⁻¹, respectively). Daytime PBLH follows the order spring (1504 m)＞winter (871 m)＞autumn (572 m)＞summer (468 m), while nighttime PBLH follows the order summer (52 m)＞autumn (23 m)＞spring (27 m)＞winter (19 m). Overall, PBLH was higher in the wet season (44 m) than in the dry season (21 m). PBLH fluctuates greatly during the daytime and remains relatively stable at night, reaching its daily peak around 1400–1500 BJT in the afternoon. PBLH can exceed 3,500 m during the dry season and 2,000 m during the wet season. Daily fluctuations are greatest in spring and during the dry season. On an annual scale, PBLH is positively correlated with 10-m wind speed, surface sensible heat flux, ground–air temperature difference, surface temperature, and 2-m air temperature, and negatively correlated with relative humidity, total cloud cover, surface latent heat flux, and 2-m dew point temperature. On a seasonal scale, PBLH in spring, autumn, winter, and the dry season was primarily driven by 10-m wind speed, while in summer and the wet season, it was primarily driven by high latent heat flux, total cloud cover, relative humidity, and low sensible heat flux. On a diurnal scale, daytime PBLH development was primarily affected by 10-m wind speed, surface sensible heat flux, and relative humidity, while nighttime PBLH was primarily affected by the surface latent heat flux.