Abstract: There are all kinds of factors can affect radar electromagnetic wave when it propagating in the atmosphere.They can result radar detect singularly.And there will be a duct-propagation in the condition of evaporation duct.This paper describe how to getting refractive index section using meteorologic data.And then get radar detection range in the condition of evaporation duct by propagation model and modified-radar equation.In the end of the paper,validate the way discussed by the experimentation in Hainan.