It is generally believed that the South China Block was formed by the collision of the Yangtze Craton and the Cathaysia Block along the Jiangnan Orogenic Belt in the Proterozoic, and then experienced multiple periods of complex magmatic—tectonic geological activities such as intracontinental orogeny and oceanic subduction. However, the geological features of the South China Block have not been well-explored so far due to the complex structurally geological environment, especially the spatial contact relationships of multiple blocks, deep mineralization characteristics, and regional geological dynamics.Methods: In our work, we performed 2D and 3D magnetotelluric inversions using the two MT profiles (i.e., 8d and 12g) collected from the programs of the crustal electrical structure of the Jiangnan Orogenic Belt and its adjacent areas, and SinoProbe project. Through the fine analysis and processing of the magnetotelluric sounding data, the lithospheric-scale 2-D MT profiles of the vicinity of the northeastern South China block were inverted using the TE- and TM- mode data with NonLinear Conjugate Gradient (NLCG) algorithm integrated in the WinGLink software. Besides, a 3-D electrical model of the study area is obtained from the inversion result of full-impedance tensor data using the ModEM inversion program, which is also based on the NLCG algorithm.Results: According to the electrical structures of the eastern Yangtze Block, the northeastern section of the Jiangnan Orogenic Belt and the Cathaysia Block in the study area, we noticed the mosaic structure caused by the collision of the Yangtze Block and the Cathaysia Block in the southwest section of the Jiangnan Orogenic Belt. The Yangtze Block crossed the Jiangnan Orogenic Belt and squeezed the Cathaysia Block at the location of the Jiangshao fault, which results in deep thrusting faults (e.g., Jiangshao fault). Although Jiangshao fault is an active fault, the contact relationship of the blocks was renewed by the invading mantle material in the northeast. The deep structures of faults, such as the Jiangshao and the Northeast Jiangxi, have been drastically modified. We propose that the upwelling of deep thermal materials is the primary reason for the deep dynamics of magmatic hydrothermal deposits, like gold and silver. According to the electrical characteristics, the Cathaysia Block is characterized by the high-resistivity upper crust and magma intruded middle and lower crust. The Tunxi—Yingtan—Anyuan and the Shangyu—Dapu—Zhenghe faults in the Cathaysia Block are deep and large faults, extended a depth of more than 50 km.Conclusions: Due to the subduction and retreat of the Paleo-Pacific Plate since the Mesozoic, the upper crust of the eastern South China Block was seriously damaged according to the result of surface intrusive rock mapping. Our resistivity models illustrate the existence of slab remelting in a large area. Also, the northeastern crust of the research area is thinned compared with the southwestern one, indicating it was modified severely by thermal disturbance caused by the subduction of the Paleo-Pacific Plate and post-modification.