Abstract: In digital MW transmission, depletion includes atmosphere absorbing decay, rain & fog scattering, F-selective decay, K-type decay and waveguide decay which gives different influence to communication, and among them, type K decay, waveguide decay and F-selective decay give the most grievous influence to micro-wave line which can't be predicted. Atmosphere absorbing decay and rain & fog decay can be predicted along with weather change. A pre-reservation of decay can be planed out according to late 20 years' forecast records. In some case, F-selective decay can be ignored, it is only square-topped decay that influemce MW transmission, of which statistic character is described by Raleigh function. The article points out that a fully consideration about the influence caused by different decay must be given, in order to keep error code rate below 1×10^-6.