The urban heat island (UHI) has a significant impact on thermal comfort, energy, and ecological security. Accurately predicting the daily maximum urban heat island intensity (UHII_max) can provide early warning for energy consumption and ensure outdoor thermal safety. Based on the local climate zone (LCZ) framework, multiple LCZs were selected in Guangzhou, and long-term local climate observations were conducted over three years. The applicability of a diagnostic equation for daily UHII_max proposed by a European scholar, which did not consider anthropogenic heat flux (AHF) , was validated based on the observed results. Furthermore, the applicability of a revised equation considering AHF was tested in Guangzhou. The results showed that the diagnostic accuracy of the original equation was acceptable, with a correlation coefficient r² of 0.63, Root Mean Square Error (RMSE) of 1.50 K, Median Absolute Error (MEAE) of 0.97 K and d of 0.60. The revised equation considering AHF yielded more accurate diagnostic results than the original equation, with a decrease of 0.12 K in RMSE, a decrease of 0.10 K in MEAE, and an increase of 0.04 in d. The differences in the validation results of the equation across different LCZs indicate that the equation should be modified according to the characteristics of each LCZ. However, incorporating AHF can improve the diagnostic performance of the equation in all LCZs. In conclusion, the revised equation can be used to predict the development of UHI and improve diagnostic accuracy in hot and humid cities.