In this paper, an innovative mathematic model dealing with the hysteresis effect of evaporation of pore solution and the nonlinear relationship between diffusivity of moisture and saturation degree of pores solution is established to describe the sulfate attack to reinforced concrete with drying-wetting cycles. The coupling effects of sulfate attack and external loading condition are investigated using a four point -bending experiment on a reinforced concrete beam and compared with the model predictions. In the experiments, ultrasonic array method is employed to detect the change of the field of modulus of elasticity due to the damage in concrete under sulfate attack coupled with loading condition. The external loading, resulting in tensile strain inside concrete matrix, accelerates the degradation process of concrete subjected to sulfate attack. The modulus variation of concrete under sulfate attack with loading exhibits two apparent stages: (1) the enhancing stage of ettringite crystallization and (2) weakening stage due to overexpansion. The experimental results verified the rationality and validity of the diffusion model proposed in this study, considering the coupling effects of external sulfate attack and loading condition.