Abstract: Numerical calculation was adopted to study the flow field in street canyon under different aspect ratios and wind speeds.The results showed that aspect ratios, wind speed and wall heating had significant effect on the flow field inside the canyon.Leeward-wall heating could accelerate internal convection inside the canyon. Windward-wall heating was conducive to displacement ventilation. Ground heating could enhance internal convection locally. Three heating ways had little impact on the flow field under the high incoming wind speed. The greater the aspect ratios, the more complex the flow field in the canyon. The research results implied a good practical reference value for urban planning.