The steam trap valve is used in thermal power systems to pour out condensate water and keep steam inside. While flowing through steam trap valves, the condensate water can easily reach cavitation, which may cause serious damage to the piping system. In this paper, in order to control cavitation inside steam trap valves, effects of sleeve parameters, including orifice diameter, installation angle and thickness, are investigated using a cavitation model. The pressure, velocity, and vapor distribution inside valves are analyzed and compared for different sleeve geometrical parameters. The total vapor volumes are also predicted and compared. The results show that the sleeve parameters have a significant influence on the cavitation intensity and cavitation vapor distributions. Specifically, the orifice diameter of the sleeve has much larger effect on each aspect than that of other two geometrical parameters of the sleeve. The improved geometrical parameters of the sleeve are determined to suppress the cavitation inside the valve. The sleeve with smaller diameter orifices, higher installation angle (maximum 80 deg), and higher thickness is recommended in practice for better anticavitation performance. The work is of significance for cavitation control and the optimization design of steam trap valves.