Global warming is expected to strengthen the hydrological cycle and further increase the frequency and intensity of extreme climate such as extreme heat and droughts. However, how climate extremes affect the start of the growing season (SOS), a key phenological phase, remains unclear. In this study, we explored the response of SOS to extreme heat and extreme drought across northern China based on the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) during 2001-2018, and further investigated how background environmental conditions affect SOS responses. Our results showed that extreme heat and extreme drought tended to delay SOS in a large proportion of northern China, and this delaying effect has intensified over the past two decades. The responses of SOS to extreme heat and drought were closely related to soil moisture in steppes and meadows, with the delaying effect of extreme heat and drought on SOS weakened in drier regions but strengthened in wetter regions. This pattern is associated with the nonlinear effects of soil moisture, which alleviates drought stress under drier conditions but elevates the risk of freezing injury under wetter conditions. Our results emphasize the importance of background environmental conditions in regulating vegetation responses to climate extremes and have substantial implications for ecosystem protection and management under increasing climate extremes in the future.