Abstract: Applying insulation coating outside the casing can reduce the attenuation of Electromagnetic measurement-while-drilling (EM-MWD) signals in low resistivity formations and increase the transmission depth of EM-MWD signals. For the purpose of obtaining the resistivity range of the formation applicable to the insulation coating and the influence of the coating characteristics on the received signal strength, a simulation model is established based on the finite element method, and simulation experiments are carried out on the application effect of the insulation coating applied to the casings of different resistivity formations and the influence regularity of the coating resistivity, thickness and length on the signal strength of EM-MWD. The results show that the insulation coating has a more significant enhancement effect on the EM-MWD signal in formations with low resistivity; with the increase of the resistivity of insulation coating outside the casing, the EM-MWD signal is first weakly enhanced, then rapidly enhanced and finally stabilized; with the increase of the thickness of the insulation coating, the received signal strength shows a trend of increasing first and then remaining stable; when applied in a low resistivity formation, as the length of the coating increases to cover the casing at the formation, the signal strength increases. The research results have theoretical significance for the rational use of insulation coating outside the casing technology to break through the well depth limit of EM-MWD application.