Abstract: Based on electromagnetic field theory, this paper derived the theoretical formulae used for crosswell formation resistivity logging in cased hole, conducted numerical simulation with low frequency electromagnetic wave, and analyzed the effects of casing on crosswell resistivity logging. It showed that it is feasible to measure the formation resistivity using low frequency electromagnetic wave between wells. The results provide a theoretical foundation for the interpretation of cased well logging data, the study of crosswell electromagnetic tomography, and the development of logging tools.