Packing ring which minimizes steam leakage generating between a rotor rotated and casing fixed is a component in steam turbine and its role is important for high efficiency in the turbine. The more reducing clearance between rotor and packing ring makes the less steam leakage, but too small clearance causes the damage of tooth of packing ring by vibration of rotor. The conventional packing ring is designed with a large clearance for preventing the damage due to vibration of rotor, and it causes an increase in the steam leakage. In this study, a flexible packing ring using winding springs was developed to prevent the damage of tooth with maintaining the smaller clearance than that of conventional design with no vibration condition of rotor. For designing the winding spring, an allowable stress and a required minimum load for the winding spring were obtained by theoretical analysis and FEA, and the shape of the winding spring is designed by applying curves at center and end parts of the flat spring. A formula for winding spring constant was obtained by using parallel spring equation, thermal deformation equation and elliptic laminated leaf spring constant equation for suspension. For verifying the formula, the results of theoretical analysis were compared to those of FEA by using a thermal-structural coupled field analysis. For predicting the leakage of flexible packing ring, CFD analysis was performed for the steam turbine under both a high pressure(HP) and low pressure(LP), and the analysis results were compared to the leakage of the conventional packing ring. To verify the leakage reduction efficiency and reliability of CFD analysis, the tests for flow rate measurement were performed. The results of CFD has a good agreement with those of the tests within 6.4% error, and the leakage reduction of the flexible packing ring showed 15.3% improvement compared to those of the contentional type.