Abstract: The existance of electromagnetic radiation before earthquakes has been proved by many recording data in earthquake monitoring at home and abroad.In order to constrain the background noise to stick out the wave shape of electromagnetic radiation before earthquakdes, the laboratory technicians developed an "electrode sensing devoce" of electromagnetic recording instrument before earthquakes based on practical experience and electromagnetic basic theory in 1988.The three years" experimental study has proved that the new sensing device reduced the background noise, struck out radiation wave shape, raised the signal noise ratio and lasted its service life.