Aiming at the fact that the distribution of evaporation duct height is easier to obtain than the original history atmospheric and hydrology data in some sensible areas, an approximate calculation method of average radar detection range of searching radar over sea from the distribution of evaporation duct height is advanced, then this method is tested. The testing result shows that the difference between this method and direct calculation is less than 20km, so this method can meet the requirement of average radar detection range and be well applied in the evaluation of radar detection ability of searching radar in sensitive areas.