In order to study the bonding shear performance of the casting interface in the functionally gradient concrete (FGC), a multiphase mesoscopic finite element modelling method was proposed to simulate a sandwich FGC specimen with the normal weight concrete (NWC) casted on the hardened ceramsite lightweight aggregate concrete (LWAC) layer. Four kinds of phases were considered during modeling: coarse aggregates, mortar substrate, interface transition zone (ITZ) between the mortar and coarse aggregates, the casting interface transition zone (C-ITZ). Each phase in the model is endowed with the respective material property based on the constitutive damage plastic model (CDP). The structural characteristic of the C-ITZ that is related to the casting construction of the FGC were fully considered in the proposed modeling method. Based on the verification by corresponding experiments, the internal damage developing and the failure mechanism of the FGC model are effectively reflected by the simulation. For the designed FGC specimen corresponding to the experiment, the weak position are proved to be located on the C-ITZ. The damage developing and damage characteristic of the given FGC specimen are more related to the casting interval time of the FGC.