Urban heat islands (UHI) pose significant impact on urban development and human life. The eastern monsoon area of China is often affected by the Western Pacific Subtropical High (WPSH) in summer, the airflow sinks in the area with WPSH control, and it is difficult to diffuse the heat on the surface of the ground, so the effect of the UHI increases significantly. While limited studies have been conducted on the relationship between WPSH and UHI at home and abroad, the mechanism of the WPSH effect on the UHI and the difference between day and night are still unclear. We established different city size classes and two conditions: with and without WPSH control, extracted the rural background by employing an urban-rural dichotomy. We used the land surface temperature data from MODIS satellites to calculate the UHI intensity and the spatial urban heat proportion index (UHPI) for cities of each size in the middle and lower Yangtze River region in July from 2013-2022, and analysed various types of influencing factors with the geodetic detector. In order to explore the UHI characteristics of different sizes cities in the middle and lower reaches of the Yangtze River with and without WPSH control. The analysis revealed findings. (1) The spatial difference of UHI is large during the daytime, the strongest heat islands significantly distributed in the centre of large cities, and weak heat islands distributed over a wide area at night. The high values of daytime and nighttime UHI intensity are distributed in large cities with WPSH control, and the high values of daytime and nighttime UHI intensity without WPSH control are in medium-sized and small cities. (2) The average nighttime UHPI values are higher than the daytime UHPI values in cities of different size classes, and the nighttime UHPI values are generally higher in the months and years with WPSH than without WPSH. Daytime and nighttime UHPI values are generally higher in super big cities and megacities than in other sizes cities, and nighttime UHPI values are higher in the two types of small cities than in medium-sized city and big city type Ⅱ. (3) Surface features have a significant effect on the nighttime UHI, with the main factor being the building index with WPSH and the vegetation index without WPSH. The air quality factors affecting the daytime UHI intensity are ozone and air quality index, and for the nighttime UHI is aerosols. The important factors influencing the intensity of the UHI during daytime and nighttime were the social activity factors of nighttime light and population density and the meteorological factor of the distance of the main body of the WPSH.