The coherent and incoherent scattering are composed in the backscattering characteristics of arbitrarily shaped dielectric object with rough surface both in laser and THz bands. If the radius of curvature at any point of the surface is much greater than the incident wavelength which is also longer than the surface height fluctuation and RMS of surface slope, the Kirchhoff approximation and Physical optics method, as well as the stationary phase evaluation are invited here to deduce the analytical expression of coherent backscattering cross section of rough dielectric object. Basically, the coherent cross section can be viewed as the combination of the RCS of corresponding smooth and perfectly conducting object, the Fresnel reflection coefficient of dielectric surface and the characteristic function of rough surface. Thus, the scattering expression of rough conducting object, smooth dielectric object and the rough dielectric object can be logically obtained. Using the tangent plane approximation, the surface of the object is divided into a series of patches, and then the incoherent component is achieved by integrating over the illuminated area combined with the covering function. Based on the Physical optics approximation and GPU parallel computing, the coherent scattering component of smooth conducting object, the incoherent component of rough object and its corresponding backscattering cross section can be easily computed. In this paper, we numerically simulate the backscattering characteristics in laser and THz bands of rough dielectric sphere and other complex rough dielectric targets respectively, meanwhile, we also analysis the influence of dielectric coefficient and roughness concentration on the results of the backscattering cross section.