Occupant’s thermal adaptation to free-running environment of apartments in summer was studied at an individual level.The relationship between indoor environmental parameters and occupant’s thermal sensation to indoor environment has been developed. Moreover, the effects of indoor temperature and relative humidity, as well as the interaction effects of the two parameters on occupant’s thermal sensation have been analyzed. On this basis, the characteristics of occupant’s adaptation to indoor temperature and relative humidity have be identified simultaneously. One particular individual dweller in Beijing has been selected to study the thermal adaptation by applying the method to practice.