Because of its high altitude, the Qinghai-Tibetan Plateau (QTP) serves as a de facto ecological barrier between China and Southeast Asia. Studying the phenological characteristics of vegetation on QTP and their response to climate change can help us to understand how climate change can impact on highland terrestrial ecosystems and how to improve predicting highland vegetation phenology. Using long-term NDVI data we derived from the MODIS spectral reflectance product (MOD09A1) and in situ climate data in QTP, we investigated (1) the trends of temporal and spatial variations in the phenological characteristics of the highland vegetation, such as the start of growing season (SOS), middle time of growing season (MOS), end of growing season (EOS), and length of growing season (LEN), and (2) the responses of SOS, MOS, and EOS to asymmetry in diurnal warming, i.e., asymmetry in the preseason daily daytime maximum temperature (T_max) and daily nighttime minimum temperature (T_min). Results showed that (1) the vegetation phenology in QTP displayed a salient zonal distribution pattern. From the northwest to the southeast, the vegetation SOS and MOS gradually advanced, EOS decreased, and LEN extended; (2) T_min warmed up faster than T_max, which indicated that daytime warming and nighttime warming were asymmetrical; (3) the increase in preseason T_max and that in preseason T_min played different roles in SOS, MOS, and EOS; and (4) the preseason T_min showed a stronger control on vegetation phenology than the preseason T_max. For instance, a 1°C increase in preseason T_min advanced SOS by 7.07d (p  <  0.05) and MOS by 6.80d (p  <  0.05), and delayed EOS by 6.70d (p  <  0.05). While a 1°C increase in preseason T_max delayed the SOS, MOS, and EOS by 5.12d, 4.84d, and 1.04d, respectively.