Different geophysical exploration methods have significant differences in terms of exploration depth, especially in frequency domain electromagnetic (EM) exploration. According to the definition of skin depth, this difference will increase with the effective detection frequency of the method. As a result, when performing three-dimensional inversion on single type of EM data, it is not possible to effectively distinguish the subsurface geoelectric structure at the full scale. Therefore, it is necessary to perform joint inversion on different type of EM data. In this paper we combine the magnetotelluric method (MT) with the controlled-source audio-magnetotelluric method (CSAMT) to study the frequency-domain threedimensional (3D) joint inversions, and we use the unstructured finite-element method to do the forward modeling for them, so that the numerical simulation accuracies of different electromagnetic methods can be satisfied. By combining the two sets of data, we can obtain the sensitivity of the electrical structure at different depths, and depict the full-scale subsurface geoelectric structures. In actual mineral exploration, the 3D joint inversion is more useful for identifying subsurface veins in the shallow part and blind mines in the deep part. It can delineate the morphological distribution of ore bodies more completely and provide reliable EM interpretations to guide the mining of minerals.