The extra high voltage(EHV) power lines are not only a power frequency electromagnetic pollution source but also a RF electromagnetic pollution source because of the partial discharge such as corona.In order to get the distribution of electromagnetic in different frequency of the EHV power lines accurately and the reasonable width of the secure aisle,the authors measured the electromagnetic environment below the 500 kV single loop EHV power lines,which are arranged in triangle and get the distribution of electromagnetic environment in different frequency.After comparing the measuring result with some electromagnetic radiant secure standardsof national prescripts,the reasonable secure aislewidth of the single loop 500 kV power lines are profosed,which are arranged in triangle,and the RF fields have been taken into account.