The thermal environment of urban street canyons is closely related to design factors. By establishing a dynamic model of street canyons numerical simulations of temperature fields for typical street canyons are conducted. Through numerical analysis the influence of the most common variables related to environment in China’s urban residential quarters is compared and contrasted. Environmental factors include the height-width ratio of the street canyon, ground paving materials, canyon directions, and facades of buildings on canyon sides. Simulation results reveal how these factors influence the street canyon thermal environment. Results also give the recommended optimal height-width ratios for urban street canyons and question and revise some design ideas prevalent in China today.