The influence of casing can not be ignored for evaluating the transmission effect of EM-MWD signal, and it is difficult to analyze this effect with mathematical models because of the casing's extremely complicated effects. Therefore, modeling and stimulation of EM- MWD signal transmission in the casing were made by ANSYS finite element software to analyze the influence rule of casing on EM-MWD signal transmission in the aspects of ground current density and ground receiver voltage. The stimulation results show that the casing plays a guiding role to the signal when insulation nipple is out of the casing, and conversely, the casing has a shielding effect on the signal; low resistivity drilling fluid has a great influence on EM-MWD signal transmission in the casing.