As an important energy generation device of the compressed air energy storage (CAES) system, the radial-inflow turbine with shrouded impeller is employed to avoid the leakage flow in the rotor, especially in the high-pressure stages. However, a lack of clarity in the leakage characteristics and their drivers still prevents a systematic approach to the efficient performance and proper design of the shrouded radial turbine. In the present work, the shroud cavity leakage of the shrouded radial turbine has been studied numerically. The physical quantity synergy is innovatively employed to research the internal flow field of the shroud cavity. It is found that the influence of high rotating speed on the seal leakage cannot be neglected, and the average reduced rate of seal leakage is found to be about 9.9% for the designed clearance. The leakage mass flow rate could be reduced by increasing the rotating speed or decreasing the seal clearance. The synergy angle is able to predict the flow resistance in shroud cavity very well. According to the volume-averaged synergy angle in the seal, the dimensionless seal clearance smaller than 1.5% in the shrouded radial turbine is recommended. Compared with the seal clearance in other high-pressure shrouded turbomachines, the current recommended clearance should be within a reasonable field.