Abstract: The transient electromagnetic method (TEM) and the direct current method (DC) are the two main roadway advancing detection method, the joint inversion between them helps to improve detection accuracy. The article presents a least squares joint inversion method using the parameter of the ratio of the logarithm of TEM response and DC response. The test of the whole space of infinite vertical tabular body showed that the joint inversion solve the equivalent phenomenon in the DC inversion and the insensitivity of the high-body in the TEM inversion. The joint inversion improves the inversion effect of the tabular resistivity and thickness obviously (with a several times to several thousand times relative error) ,besides, the distance between the tabular body and the head and its surrounding medium resistivity accuracy is futher improved.