Scramjet propulsion is a promising hypersonic airbreathing technology that can enable flexible and economical systems for access-to-space and atmospheric cruise in both civilian and strategic applications. Fuel injection and efficient mixing play a crucial role in scramjet operation, which depends critically on the sequential flow process, particularly for scramjet configurations featuring upstream fuel injection. However, designing high-performance injection systems represents a substantial challenge to conventional design approaches due to complex flowfields introduced by highly coupled aerodynamic phenomena. The present study is conducted by applying an advanced methodology combining computational fluid dynamics and evolutionary algorithms assisted by surrogate modeling to a multi-objective optimization problem for high-performance injector design, assuming scramjet operation at Mach 7.6. Optimization is performed for elliptical injector configurations defined by four design parameters, namely, injection angle, spacing, aspect ratio, and hole radius, simultaneously aiming to maximize three objectives, that is, fuel/air mixing, total pressure recovery and fuel penetration into air. Flowfields are scrutinized for selected injector configurations and global sensitivity analysis is applied to the surrogate models trained during the optimization, in order to gain physical insight into underlying flow mechanism and to identify key design factors for mixing enhancement. It has been found that the injection angle and aspect ratio are primarily responsible for fuel/air mixing efficiency, while fuel penetration largely depends on the injector spacing in conjunction with the injection angle.