Many Chinese people leave big cities for family reunions during the Chinese New Year (CNY), which is the most important public holiday in China. However, how modern mass human migration during the CNY holiday affects the urban heat island (UHI) is still unknown. Here, we investigate the role of modern human migration for the UHI effects during the CNY holiday for the period of 1992–2006 in Harbin City, Northeast China. The results show that during the CNY week, the UHI effects expressed as daily mean, maximum, and minimum temperature differences between urban and rural stations averaged over the period of 1992–2006 are 0.65°C (43%), 0.31°C (48%), and 1.14°C (71%) lower than during the background period (four weeks before and four weeks after the CNY week), respectively. Our findings identify previously unknown impacts of modern mass human migration on the UHI effects based on a case study in Harbin City.