Haikou is located in tropic climates in China. The outdoor environment in summer is too harsh to stay here. However, the traditional architecture, namely arcade, still survives in the severe climate. Its indoor and outdoor parameters of arcade are tested in field to research its thermal environments. The heat stress index is used to study the adaptability mechanism of arcade and local human to hot and humid environment. It proves that the metabolic rate and wind velocity are main and adjustable factors to reduce heat stress in such wretched weather. On the condition of metabolic rate below 60met and wind speed above 0.7m/s, the healthy adult can work normally in the arcade or on the corridor. So the human in tropic area should keep their metabolic rate and period staying out in the summer daytime as less as possible, and the architects should make full use of the local wind speed to organize natural ventilation.