Human health and social development are significantly affected by urban extreme heat. It is a new proposition for human-land relationship in the field of geography to measure the characteristics and social consequences of urban extreme heat. Applying the tool of social vulnerability to studies of urban extreme heat, this paper takes 296 cities in China as research objects and establishes quantitative indicators of urban extreme heat such as high temperature days, high temperature intensity, heat wave frequency, heat wave duration and heat wave intensity. By using daily maximum temperature data, urban statistics and census data, we systematically analyze the characteristics of urban extreme heat. Meanwhile, we construct a framework for urban social vulnerability to extreme heat and based on this framework, we developed a common evaluation index system of social vulnerability according to the three dimensions of exposure, sensitivity and adaptive capacity. Finally, we conduct social vulnerability assessments for the 296 cities, classify social vulnerability levels, and analyze the causes of urban social vulnerability. The results are shown as follows. (1) The extreme heat events are mainly concentrated in southern cities, especially in the eastern and central parts of the country. Although there are fewer extreme heat events in northern cities, the intensity of high temperature is more prominent. (2) The urban exposure index to extreme heat in China has obvious spatial agglomeration characteristics, while the sensitivity index and adaptability index are scattered. (3) Cities with high social vulnerability index are mainly concentrated in most areas of East and Central China, and in a few areas of Southwest and North China. The proportion of cities with high, middle and low social vulnerability index was 25.3%, 46.3% and 28.4%, respectively. (4) The number of social sensitive cities is the largest, followed by high temperature exposure cities, and the number of insufficient adaptability cities is the smallest. In addition, exposure index contributes the most in cities with high social vulnerability index, and sensitivity index contributes the most in cities with middle and low social vulnerability indexes. This study can provide reference and enlightenment for relationship research between disaster and society, quantitative expression of characteristics of urban extreme heat and assessment of social vulnerability to extreme heat.