The frequency of the exciting current affects the design of hardware circuit and quality of reconstructed images in electromagnetic tomography. According to the skin effect and skin depth formula of high frequency current, the law of the exciting frequency for different test objects is concluded. The advantage of electromagnetic tomography is introduced simply. The relationship curve between the exciting current frequency, conductivity and the skin depth is draw, from which the frequency scope for common test object is given. With the same test object, the same position, and the same orientation, different reconstructed images are obtained in different exciting current frequencies. It can draw a conclusion that the metal medium need the low frequency to excite, KHz or smaller, on the contrary, biological tissues and moisture contents need high frequencies, MHz or higher.