Electromagnetic measurement while drilling can provide large amount information with high transmission rate, this technique was rapidly applied and developed in recent years. The paper summarized the basic modes of electromagnetic signal transmission in downhole drilling, analyzed the structure parameters of downhole emitting electrode by equivalent transmission line with primary test. The results could only partly prove validity of the verification theory; other parts of parameter calculation could not get the reasonable explanation by theoretical model, which need further improvement to understand drilling rod / magnetotelluric transmission system.