With the acceleration of urbanization, the problem of the urban heat island effect has become increasingly prominent, seriously impacting on the quality of life and health of urban residents. This study used mobile observation method to measure the thermal environment of the street canyon in Kunming’s Chenggong District. And select nine street samples with similar lengths to analyze the impact of street canyon orientation and aspect ratio on the thermal environment. Results show that in the morning, north–south oriented streets are about 1.5 °C warmer than east–west oriented streets with the same aspect ratio. In the afternoon high-temperature period, the north–south oriented streets are about 0.9 °C cooler than the east–west oriented ones, indicating better thermal comfort for north–south oriented street canyons compared to east–west oriented ones. Additionally, the relationship between the street aspect ratio and street temperature is associated with solar irradiation, showing a positive correlation in the morning and a negative correlation in the afternoon, with this correlation also being influenced by the street orientation. This study provides a scientific basis for livable urban planning and the optimization of urban microclimate environments.