Taking cross-sea and cross-river 220 KV power transmission and transformation project as the research object, and through field observation and theoretical analysis, mathematical model of power frequency electric field of EHV (extra-high voltage) overhead transmission lines is established. This paper presents the calculation method of electric field intensity around the transmission lines, and analyzes the effect of electromagnetic radiation on radar reflection and the characteristics of radar echo. Finally, according to the research results, measures and suggestions on correct operation and use of marine radar are put forward.