Based on the nonlinear temperature field theory, the temperature program of reinforced concrete member was developed. By analyzing the mechanical behavior of different temperature-stress specimens, the transient strain importance was discussed, and the transient strain model under the biaxial stress state as well as transient modulus was proposed. According to the flat shell theory and thermal elastic-plastic constitutive model, the finite element model of reinforced concrete two-way slab was established to simulate the fire behavior of the three slabs. In addition, the effect of transient strain and thermal strain on the fire behavior of the slab was studied. The results have shown that the predictions agree well with the test results, and the transient strain model is verified. The parametric study indicates that the transient strain and thermal strain have an important influence on the fire behavior of the slab.