Tip-leakage loss caused by tip-leakage flow is an important source of aerodynamic loss in turbine rotor. Squealer tips are often used to control the tip-leakge flow and loss. In this paper, a transonic single-stage high-pressure turbine is simulated numerically to investigate the effects of cavity width and height on tip-leakage flow and loss. Based on plenty of cases with various cavity widths and heights, it is found that the optimal value of cavity height is 2.5-3.0 times clearance height and the optimal value of cavity width is affected by cavity height. The decrease of cavity width and the increase of cavity height of cavity have similar effects on the evolution of the scraping vortex. The cavity width controls the tip-leakage loss by suppressing the breakdown of the tip leakage vortex and reducing the corresponding mixing loss. However, the cavity height mainly affects the loss inside the clearance.