Based on electromagnetic field theory, the paper has studied the property of the signal propagation and attenuation of electromagnetic wave for MWD transiting through strata. Some conclusions have been drawn as follows. Firstly the signal attenuation will be increase with the decrease of the stratum resistibility. Secondly in the frequency range form 1 to 10 Hz the stratum absorptivity is tiny and does not add noticeably with the increase of the electromagnetic wave frequency and the decrease of the stratum resistibility. Thirdly in the frequency range from 1 to 10000 Hz the stratum absorptivity does not increase noticeably with the decrease of the dielectric constant of the stratum. The conclusions as mentioned above have provided the theoretic foundation for the design, study and application of the electromagnetic wave MWD.