Time domain electromagnetic (TDEM) method is widely used in detecting subsurface objects such as pipelines, unexploded ordnance (UXO) and archaeological studies. But the characteristics inversion of anomaly is still a challenging problem which causes the increase of false alarm rate and cost of labor. This paper proposes a threedimensional orthogonal equivalent magnetic dipole model and a target parameter synthesis method. With the proposed model and method, the target size, material and shape parameters can be synthesized by the time attenuation characteristics of the equivalent magnetic dipole. The proposed model and method is tested by the developed TDEM system. Experimental results indicate that the model can reproduce the response of subsurface objects, and the parameters of the dipoles can be used to estimate the characteristic of the object. These features can lay solid foundation for identifying subsurface objects.