Abstract: The tensile membrane theory for the cracking state of reinforced concrete (RC) slabs does not explain the mechanism that enables the slabs to bear the load under a large deformation before the cracking at slab bottom. Therefore, three-dimensional solid finite element (FE) models of simply supported one-way RC slabs and two-way RC slabs in both heat transfer analysis and thermo-mechanical coupling analysis were established using the ABAQUS software, based on reasonable thermal parameters and thermo-mechanical coupling constitutive of materials. Using the verified model, the stress variation law of concrete and steel bars and the mechanical response of simply supported RC slabs under fire were further investigated. The results show that: simply supported RC slabs undergo intense stress redistribution, and their responses show four stages, namely elastic, elastic-plastic, plastic and, tensile cracking stages. In elastic-plastic and plastic stages, the bidirectional compression of bottom concrete, namely inverted arch effect, makes the fire resistance of two-way RC slab excellent; there is no cracking in the fire area of the slabs until the tensile cracking stage; Compared with one-way RC slabs, the time for two-way RC slabs to enter the plastic and tensile cracking stage is postponed, and the deformation rate in the plastic and tensile cracking stage is also slowed down. Therefore, the two-way RC slabs have better fire resistance.