This paper proposes a conceptual design for a water-air trans-medium aircraft capable of underwater gliding, surface takeoff/landing, and high-speed aerial flight. Considering the significant differences between aquatic and aerial environments, the aircraft’s concept parameters require integrated optimization based on dual-medium characteristics to enable multimodal operation. Through flight profile modeling aligned with mission scenarios, parameter-mission capability mapping relationships are established. The study systematically analyzes the correlations between thrust-to-weight ratio, wing loading, and three key performance metrics: aerial cruise radius, surface take-off efficiency, and underwater gliding distance. The results indicate that aerial cruise radius exhibits a direct proportionality to wing loading and inverse proportionality to thrust-to-weight ratio. Surface maneuverability shows minimal dependence on wing loading but significant sensitivity to thrust-to-weight ratio. Underwater gliding performance primarily correlates with thrust-to-weight ratio, demonstrating weaker association with wing loading variations.