Abstract: In electromagnetic measurement while drilling (EM-MWD) systems, drill strings are important channels for EM-MWD signal transmission. For a better understanding of the factors that influence the EM-MWD signal transmission effect in drill strings, the equivalent transmission line method was used to model the EM-MWD signal channel, and the finite element calculation was made using ANSYS software. With the analysis of strata division, research was conducted on the energy loss of signal transmission by drill strings with different specifications in an alternating electromagnetic field and the influence law of casing on EM-MWD signal transmission. Several findings were obtained in this paper. The increase in power and frequency would enhance the energy loss of EM-MWD signal in drill strings, and when the signal frequency exceeded 100 Hz, the loss grew more evidently. The transmission effect varied for different drill strings, but it was better when the ratio of drill string wall thickness to outer diameter was 0.08–0.20. Adding casing on the drill string would shield the transmission of EM-MWD signal and affect the transmission effect. However, when the frequency of downhole EM-MWD signal was below 50 Hz, the impact of casing is small. The research results further clarified the influence of the drill string's own properties on the EM-MWD signal transmission, which can provide a reference for the improvement and design of the EM-MWD system.