The finite-difference time domain (FDTD) method is widely used in ground–airborne transient electromagnetic modelling. Generally, this method offers great limitations because the calculation of the air layer is avoided and the surface assumption is Cat. This study proposes a new method for ground–airborne 3D electromagnetic forward modelling that includes air layers and effectively solvesthe airborne TEM forward calculation of the coastal ocean and rugged terrains. The optimum selection of the air-layer resistivity and the identification ability of the long-wire source are discussed. By adding the pulse source signal to the control equation and realizing a direct calculation that includes the source, the cumbersome process of calculating the complex initial field is avoided. After establishing the mountain model and coastal ocean model, the electromagnetic response is discussed. Subsequently, the ground–airborne TEM modelling is subjected to a 3D numerical simulation of complex geological conditions. The validity of this method is verified by analytical solutions through a uniform half space. The results show that the method can achieve 3D and high-precision numerical calculations when it includes complex terrain and seawater.