Natural ventilation (NV) has been considered a simple and effective method of ventilation. However, the introduction of NV does not achieve better indoor air quality (IAQ) when the outdoor atmospheric environment is polluted. Therefore, portable air cleaners (PACs) are increasing in use in recent years, but their effectiveness is highly dependent on the residents' habits. A typical residence in Xi'an, China was selected to examine the effects of the use of NV alone and the use of NV and PACs together on IAQ in the three occupant states, i.e., unoccupied, sleeping and leisure. Parameters, such as temperature, relative humidity, CO2, and PM2.5 concentration were measured when changing the window opening and the position of the PAC. The results showed that in the unoccupied state, opening the inner door can promote a more uniform thermal and humid environment. In the sleeping state, the I/O ratio of the PM2.5 concentration was the lowest when the window opening of the bedroom was 1/2 or 3/4, with a mean value of 0.3. In the leisure state, only using NV, when the purification rate reaches 90%, the mean purification time of each window opening in the living room is 87.5 min. The mean purification time was reduced to 25 min when both NV and PAC were used. The on-site purification efficiencies were 91.0% and 94.5%, when the window opening was 1/2 (i.e., the PAC was placed in the center of the room) and 3/4 (i.e., the PAC was placed away from the outer window), respectively.