Abstract: The condition of the lower crust in a region at a certain time is a question that has not been considered by academia, for the reason that there is no means. Our recent research on the origin of granite shows that granite is derived from partial melting of lower crust metamorphic rocks, and the granite is in equilibrium with the melt residual phase. Therefore, we can invert the characteristics of metamorphic rocks in the lower crust based on the characteristics of granite at the surface. On the other hand, the progress in the research of metamorphic rock indicate that it is possible to construct apparent profiles of metamorphic rocks of different compositions according to different temperature and pressure conditions, and to infer the properties of the melts formed during partial melting. The combination of these two aspects forms the theoretical basis for the mapping of the lower crustal, based on which to understand the conditions at the bottom of the lower crust, and thus the mapping method of the lower crust is derived. This paper discusses the method of mapping the lower crust, discusses the content of the lower crustal geological map that may be expressed in the current situation, focuses on the lower crustal information can be given by granite, and puts forward the principles for dealing with controversial issues. It also discusses how to map the lower crust in the absence of granite outcropping, and the information of the lower crust that may be provided by sedimentation, stratigraphy, paleontology, minerals, and structure are discussed, and the vitrinite reflectance method is recommended. In addition, the significance and limitations of lower crustal mapping are also discussed. At last, we discussed the function and significance of lower crust mapping by taking the geological map of the lower crust in the Early Cretaceous of Shanxi Province as an example.