Diffusing double-S inlets are of particular interest in the aircraft design due to their ability of reducing the radar signature and drag. Surface composition of the compact and offset bend douhle-S inlet can he quickly accomplished with a polynomial central line, along with complex conformal intake, midsection and outlet shape. Six ducts are created and multi-parameters which include centerline curvature, area ratio and middle-cross-sec- tional shape are analyzed utilizing that method to study their effects on duct performance. Results indicate the approach of a polynomial central line and cross-sectional shape is effective in the double-S inlet design and that there are more complex flow structures in the double-S inlet compared with that in the S-duct inlet. Great emphasis should be placed on the control to the flow separation lying about the second S-bend and flow distortion arose by eddy, those parameters of central line curvature, expansion angle and polynomial should be chosen to beneficial the flow stability of the second S-bend.