Because of high heterogeneous of cities,studying the intra-city thermal environment difference is of great importance for the deep understanding of urban residents' thermal comfort and making heat stress mitigation measures. This study conducted four-season microclimate mobile observation experiments in Lanzhou, a semi-arid city, to get wet-bulb temperature data and further analyzed the spatio-temporal distribution characteristics of humid heat and investigated the synergistic relationship between temperature and humidity and their contribution to wet-bulb temperature.It also explored the impact of underlying surface types, urban morphological characteristics, and human activities on humid heat using multi-source data. The main conclusions are as follows: (1) there are significant diurnal variations in the spatial distribution of humid heat in Lanzhou, but seasonal differences are not significant. (2) In the daytime, the wet-bulb temperature is more likely to be higher than that at night, mainly due to the synergistic effect of temperature and humidity during the daytime. (3) Shading by buildings effectively reduces urban humid heat in the daytime, while urban green spaces at night can significantly alleviate humid heat. (4) Given the notable diurnal variations in the ability of urban green spaces to mitigate humid heat under typical weather conditions, future studies should consider the potential effects of green spaces on wet-bulb temperature during heatwaves, especially when they are constrained by moisture stress and other adverse conditions.