Wet bulb globe temperature (WBGT) is the thermal environmental index that acquired the ISO7243 authentication of the international standard system,which is adopted by countries including America, Australia, Japan and China, etc. Currently, most of the simplified calculation of the outdoor WBGT is summarized or verified based on the actual measurement in low altitude areas. To research the simplified calculation method of outdoor WBGT for areas with low latitude and high altitude, taking Guiyang, a typical city with low latitude and high altitude, as an example, we conducted Pearson correlation analysis and hierarchical regression analysis on the air temperature, solar radiation, relative humidity, wind speed, air pressure and WBGT of different altitudes in summer. The result shows that except the wind speed, other four parameters are of linear dependence with WBGT, and can be brought into the regression model. In the hierarchical regression, adding in air pressure can improve the prediction approach of air temperature, solar radiation and relative humidity to WBGT, the regression model has statistical significance and so does the influence of the four independent variables added into the model on WBGT. In the outdoor thermal environment evaluation and actual measurement, empirical formulas need to be verified first when adopted in different regions, and it is suggested to add the air pressure parameters in WBGT calculation model to improve the prediction accuracy at high altitude.