Scramjet engines are a type of air-breathing engines which work in hypersonic flows. Streamline tracing is an effective approach to design efficient and robust three-dimensional intakes for scramjet engines, a hypersonic airbreathing propulsion technology that offers promise for economical and flexible access-to-space. An effective application method of this technique and the design strategy of the base flowfield are examined by means of multi-objective design optimization. Consequently, the most influential geometrical parameter is has been identified. In perticular, the contraction ratio which is mainly determined by the location where streamlines are sheared off, has large influence on the performances. This result represents the importance and effectiveness of optimization for the application of streamline tracing.