With the attention to the comfort degree of living environment, the phenomenon of making use of the PMV -PPD index within the ISO7730 standard to evaluate the indoor thermal comfort degree is more and more widespread. Many papers about indoor thermal environment are based on the PMV - PPD index as the reference or basis to research and analyze. But few people understand and pay attention to the application range of the PMV - PPD index. So some people misuse and abuse the PMV index and deduce some inaccurate conclusions. In this thesis, based on the analysis of the process of deducing PMV equation, the phenomenon that PMV index is different from the fact in the high temperature condition is explained; furthermore the application range of the PMV equation is confirmed. So to obtain the scientific and reasonable evaluation and conclusion in the process of research of the thermal comfort degree can be ensured.