Combined with domestic and international research on the mechanical properties of concrete and steel after fire, a three-step model and a two-step model were used to calculate the elastic modulus of concrete and steel simply, respectively. According to the simplified calculation model and the equivalent elastic modulus method, an equivalent cross-section elastic modulus of reinforced concrete beams was obtained after fire. Based on the effective moment of inertia method, the cross section of the reinforced concrete beam was equivalently translated before and after the reinforced concrete beam cracked. Area of the whole cross-section mortar and the reinforced area of tension zone were translated to the effective area whose modulus of elasticity is the concrete modulus of elasticity. Through the experiment data, design formula of the stiffness and deflection of the reinforced concrete beam with high-performance ferrocement laminate after fire was deduced and verified. The calculation results are in good agreement with the experimental ones.