This paper focuses on the developing rule of elastic modulus of Fly Ash Concrete (FAC) under sustained load. Mechanism of theaction of sustained load on concrete is discussed and a new model for the elastic modulus of FAC is constructed based on the TriplephaseSpherical Model (TSM). An experiment with three concrete mix designs and five testing groups were conducted. Theexperimental compressive strength and elastic modulus at six different ages were used to calibrate the parameters in the model. Influences of two important parameters (loading age and loading level) in the model were analyzed. Test data confirmed the fact thatelastic modulus for the loaded concrete have a small increment compared to the non-loaded, which is more evident for youngerconcrete, and calculation results showed that the newly constructed model can well predict the elastic modulus of concrete undersustained load. Besides, compared to the loading age, the loading level was proved to be a more important factor in deciding thedevelopment rule of elastic modulus for the loaded concrete.