With the ongoing global climate change, extreme heat events have a profound negative impact on the socio-economic development of cities. In this study, through a portrayal of the urban vulnerability pattern of cities under extreme heat stress in China, as well as an in-depth analysis of its influencing factors, a foundation was laid targeting reduction in the risk of extreme heat event occurrence in these cities and building a healthy urban environment. Herein, the municipal districts of 280 cities in China were considered as the research targets. On the basis of meteorological observation data, census data, and socio-economic statistics, the urban vulnerability assessment index system for extreme heat was constructed from the standpoint of three dimensions: Exposure, sensitivity, and adaptability. The set pair analysis method and the vulnerability index model method were used to comprehensively calculate the indices of urban exposure, sensitivity, adaptability, and vulnerability to extreme heat. Thereby, the spatial and temporal distribution patterns of urban exposure, sensitivity, adaptability, and vulnerability to extreme heat in the main cities in China for 1990, 2000, and 2010 were revealed, following which the factors affecting the urban vulnerability to extreme heat were discussed. The following conclusions from three standpoints were ultimately drawn. First, from 1990 to 2010, the urban exposure index to extreme heat in China gradually increased, the urban sensitivity index first rose and then declined, and the adaptability index demonstrated a slow decline. The overall spatial distribution of urban exposure to extreme heat is high in the south and low in the north, while the urban sensitivity and adaptability to extreme heat are widely distributed and scattered throughout the country. Second, from 1990 to 2010, the number of cities with a high vulnerability to extreme heat showed an overall trend of increase, accounting for 24%, 28%, and finally 62% of highly vulnerable cities, which implies that the vulnerability to extreme heat in China gradually increased. From 1990 to 2000, the pattern of urban vulnerability to extreme heat in China changed slightly. In 2010, the number of cities with high vulnerability increased sharply, and the distribution range expanded from the south to the north of China. Further, the overall distribution of urban vulnerability to extreme heat was high in the south and low in the north. Third, urban vulnerability to extreme heat is mainly affected by the regional climate conditions, natural environment, and differences in urban development levels. The average monthly temperature in summer and heat island intensity both have a significant positive impact on the urban vulnerability to extreme heat. On the contrary, the per capita local fiscal revenue, average precipitation in summer, output value of the secondary industry as a percentage of GDP, elevation, maximum wind speed, and per capita investment in fixed assets all have a significant negative impact on the urban vulnerability to extreme heat. However, the positive impact of the average monthly temperature in summer on urban vulnerability to extreme heat is far greater than the negative impact of social and economic conditions. Therefore, it can be inferred that urban vulnerability to extreme heat is mainly affected by the regional temperature environment.