The invention relates to a simultaneous inversion algorithm for dielectric constant and thickness of a layered medium. The Fresnel's law and the ray tracing principle are utilized to accurately calculate the values at signal travel time under transceiving offset distances of different antennae, in consideration of refraction properties of electromagnetic wave in different media. For the detection for multilayer layered medium, the calculated electromagnetic parameter of the last layer is known, the dielectric constant and thickness of the multilayer layered medium are iterated and inverted and the problem of insufficient inversion precision of the traditional speed spectrum inversion algorithm for multilayer layered medium can be solved.