The invention discloses a ground penetrating radar large-scale three-dimensional forward modeling method based on the FDTD. According to the method, data exchange of nodes is carried out through a network, and communication and synchronization of the nodes are achieved; a measured point in any node is selected to be subjected to forward modeling parallel calculation on the basis of the FDTD, firstly, an electric field value is calculated, magnetic field calculation is carried out by adding an excitation source after electric field calculation is finished, iteration is continuously carried out through communication, electric field calculation and magnetic field calculation, and when calculation of the set time total step length is finished, calculation is stopped, output data are stored, the data serve as a radar reflected signal of the measured point; initialization updating is carried out on all field values, a whole computational domain recovers to the state before calculation, and then a transmitting antenna and a receiving antenna are moved to a next measured point to start forward modeling calculation again; if calculation of all the measured points is finished, data of all the nodes are collected to carry out imaging and image postprocessing. The ground penetrating radar large-scale three-dimensional forward modeling method based on the FDTD is applicable to calculation of dielectrics and models with complex scales, has the advantage of being high in universality, and is especially applicable to large-scale and high-precision forward modeling.