Many aquatic animals are able to leap out of water effortlessly, however, it is still exceedingly challenging for a swimming robot. Inspired by the fast-swimming mechanism of fish, in this letter, we develop an untethered high-speed swimming robot with the integration of high-frequency oscillation and compliant passive components for performance improvements in swimming speed, efficiency and even fish-like leaping motion. The stiffness optimization of compliant components is conducted via experiments. As a result, both swimming speed and efficiency have been significantly improved and the robot surprisingly reaches a swimming speed of 1.88 m/s corresponding to 7.1 body lengths per second (BL/s). That is, the compliant components’ stiffness is of great importance to the performance improvement of a swimming robot at high-frequency oscillation. Additionally, the leaping motion is successfully performed with the optimized swimming robot and the maximum height of center of mass (CM) can reach up to 0.23 m. A simplified model is developed to analyze the leaping height with different speeds and body lengths, which suggests that the robot with small size leaps out of water more easily. The obtained results in this letter will offer some significant insights into the design and optimization for a high-speed swimming robot, as well as the cross-domain motions between water and air.