The paper examined the outdoor thermal conditions and analyzed the influencing factors on human thermal comfort of Changsha in summer. Both the micrometeorological measurements and questionnaires were adopted to access the empirical data. The study found that the acceptability of outdoor environment is only 77%. The neutral temperature is 29.2 degrees C and acceptable range is 25.5-32.8 degrees C, both higher than the ASHRAE standard and previous research on indoor thermal comfort, providing a reference for the designing and improvement of outdoor environment.