The five-parameter model is proposed and the parabolic equation numerical model of electromagnetic waves propagation is used to calculate the propagation loss of electromagnetic waves, then GA （genetic algorithm） is used to retrieve the duct parameter and the parameter of GA are discussed. The ability of restraining noise of retrieval algorithm is tested. Experimental results show that when the observation errors are less than 10%, the retrieval precise is credible, but when the observation errors are larger than 10%, the retrieval precise is bad. These results can provide useful information to practice inversion and radar designing.