In this paper, the low-frequency electromagnetic field in the inhomogeneous model of casing borehole was analyzed by finite element numerical simulation method, which was emphasized on the effect of resistivity, magnetic inductivity, thickness and radius of casing on electromagnetic field in cased borehole. The results show that the amplitude and phase of the received electromotive force (EMF) strengthen versus the increase of resistivity, when the resistivity reaches to some degree, the curve of EMF approximates to horizontal. Along with the increase of the thickness of the metal casing, the EMF gradually decreases and almost linearized Along with the increase of radius of metal casing, the amplitude of EMF firstly gradually decreases and then gradually increases; the phase of EMF gradually increases and nearly linearized in the semi-logarithmic coordinates.