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Combining a phase change material (PCM) and night ventilation provides a good opportunity to reduce the load of air conditioning in buildings, particularly for a large diurnal temperature fluctuation. This study developed a multiple-relaxation time lattice Boltzmann method to numerically investigate the heat transfer characteristics and energy-saving potential of porous bricks filled with paraffin waxes under a large diurnal temperature fluctuation. A typical day in July for Shanghai, Urumqi, Hohhot, Harbin, and Xining was selected, and its meteorological parameters were deduced using Meteonorm software as the dynamic boundary conditions. The total heat flux from the roof, temperature fluctuation of the interior surface of the roof, and frequency of thermal discomfort (FTD) were calculated. By comparing the results in Shanghai and Urumqi, the impact mechanisms of the diurnal temperature fluctuation on PCM performance were shown in detail for the first time. At the phase transition temperature of 28 °C, the FTD is 7.4% in Urumqi, much lower than 37.2% in Shanghai. The adaptability of the phase transition temperature to climatic parameters is highly significant for the application of PCM in building roofs. The porous brick roofs in Shanghai, Urumqi, Hohhot, Harbin, and Xining exhibited excellent operating characteristics when paraffin waxes with phase transition temperatures of 28 °C, 28 °C, 26 °C, 28 °C, and 24 °C, respectively, were employed. The corresponding FTDs were 37.2%, 7.4%, 0%, 0%, and 0%, and the corresponding total heat fluxes from the roof were 163.3 kJ·m−2, 95.2 kJ·m−2, 23.0 kJ·m−2, 61.6 kJ·m−2, and 0 kJ·m−2. This work can provide guidance for optimizing the combination of PCMs and night ventilation in buildings located in areas with large diurnal temperature fluctuations. [ABSTRACT FROM AUTHOR]