Abstract: In this paper, the menbership functions are established based on the fuzzy sets theory and the properties of preliminary parameters of aircraft. An index of the menbership function is proposed to express quantitively to what extent do a certain plan or property belongs to the "satisfactory" set. Then, the caloulating formula for synthetic evaluation is set up by the use of weighting function method, and the optimal preliminary parameters of aircraft are obtained by means of unrestrictive optimization method.An illustrative example is also given in this paper, and its result is satisfactory.