Urban heat island (UHI) is a very important local climate phenomenon. With the rapid development of global warming and urbanization, UHI effect has received more attention and research. Previous studies have shown that population aggregation and population size have a significant impact on UHI effect and regional urban climate. However, whether and to what extent the massive human migration can affect the urban climate remain unclear. The world's largest annual human migration, i.e. the population movement around the Chinese New Year (CNY), is taken as an example in the present study to investigate the effects of massive human migration on UHI during the CNY holiday for the period of 2000-2015 in Shanghai. It was found that the daily maximum (△UHⅡ_max), mean (△UHⅡ_mean), and minimum (△UHⅡ_min) surface temperature difference between urban and nearby rural stations averaged over the period 2000-2015 during the CNY week were 0.35℃, 0.90℃, and 1.53℃, respectively, which were 0.16℃ (31.4%), 0.11℃ (10.9%), and 0.15℃ (8.9%) lower than those during the background period (2-4 weeks before and 2-4 weeks after the CNY week), respectively. Our findings highlight the important role of massive human migration on urban climate based on a case study in Shanghai. Meanwhile, the results also indicate that the influences of human migration on urban climate are complex, which are often intertwined with local background climate, weather condition, city size, and the characteristics and pattern of human migration and even firework prohibition (by means of impacting the aerosols, cloud cover, and waste heat release). Therefore further studies on this issue are necessary.