A direct correction method for estimating the outdoor mean radiant temperature (MRT) with black globe thermometer method was proposed to solve the observation defects of the traditional black globe thermometer method and improve the estimation accuracy of the outdoor radiant environment in hot summer and warm winter zone. First, the six-directional radiation method was used to observe and analyze the radiation characteristics of three different underlying surface sites in Guangzhou, and the effect of the underlying surface albedo on the radiation environment was clarified. Then, the estimation results of the six-directional radiation method and the black globe thermometer method were compared, and the suitable response periods of 50 mm and 150 mm black globe thermometers were determined to be 5 min and 20 min respectively. Finally, the regional applicability of the direct correction method was verified. On the basis of the statistical results of the two observation methods for estimating MRT, a polynomial correction model was constructed for two specifications of the black globe thermometer method under different weather conditions, and the accuracy of the model was verified in the corresponding weather. Research results show that the direct correction method was suitable for regions with the same climatic conditions, and the fitting equation, the coefficient of determination (R²), and the root mean square error (RMSE) between the six-directional radiation method and the black globe thermometer method modified according to the regression function of different weather conditions were greatly improved. The deviation ranges of the two specifications of black globe thermometers were improved by 61.35% and 68.71% respectively. This study can provide a theoretical basis for the estimation of outdoor radiation environment in hot summer and warm winter zone represented by Guangzhou.