Different kinds of thermal indices have been applied in several decades as essential tools to investigate thermal perception, environmentally thermal conditions, occupant thermal risk, public health, tourist attractiveness, and urban climate. Physiologically equivalent temperature (PET) has been proved as a relatively wide applicable thermal indicator above other thermal indices. However, the current practical PET performs a slight variation influenced by changing the humidity and clothing insulation. The improvement of the PET has potentiality for further multi-application as a general and consistent standard to estimate thermal perception and tolerance for different studies. To achieve the above purpose, modified physiologically equivalent temperature (mPET) is proposed as an appropriate indicator according to the new structure and requirements of the thermally environmental ergonomics. The modifications to formulate the mPET are considerably interpreted in the principle of the heat transfer inside body, thermo-physiological model, clothing model, and human-environmental interaction in this study. Specifically, the mPET-model has adopted a semi-steady-state approach to calculate an equivalent temperature refer to an indoor condition as the mPET. Finally, the sensitivity test of the biometeorological variables and clothing impact proves that the mPET has better performance on the humidity and clothing insulation than the original PET.