In order to elucidate the overpressure and fire hazard effects of gas explosion in a congested room, the effects of gas concentration and room layout of gas explosion in a kitchen were studied by CFD. The results showed that the flow field parameters in a kitchen exhibited an initial increase followed by a decrease as the gas concentration increased. The maximum gas flow rate recorded within the chamber was 390 m/s, while the corresponding maximum flame propagation rate and peak pressure reached 289.86 m/s and 30.95 kPa, respectively. The difference in the flow field induced by the concentration is further enhanced by the presence of congested materials. Additionally, the room layout influenced the gas congestion’s blowout effect due to variations in turbulence intensity and flammable gas volume caused by significant changes in the congestion’s size within the room. Specifically, when gas concentration is 10%, the indoor gas flow rate and flame combustion rate are II > U > L > I, while the turbulent kinetic energy and explosive overpressure followed the order of I > II > L > U. The results are of great significance for the disaster assessment and accident prevention of natural gas explosion in civil kitchens.