Observation weight is generally not considered in the works on inversion ocean duct using radar echo. In order to solve the problem，an exponential expression similar to that of the electromagnetic wave propagation loss is used to describe change of observation weight with distance，and the standard statistics retrieval algorithm is used to test the effect of retrieval speed and precision caused by different observation weight. Experimental results show that the exponential expression can approprielly describe change of observation weight with distance，and the determination of loss index is of great importance. Reasonable observation weight can eviclently improve the retrieval speed and precision，and the optimal loss index is different for electromagnetic waves of different frequency.