Thermal and hydraulic performance of the converter transformer winding is essential for improving the reliability and economy of the oil-cooled converter transformer. In this paper, a full-scale axisymmetric numerical heat transfer model was developed for a unilateral valve winding of an oil-cooled converter transformer. This model was first validated using data from the reference. Effects of the oil inlet/outlet position and the block washer number on the oil velocity and disc temperature of the winding were then studied under different oil mass flow rates. The thermal, hydraulic and comprehensive performance of the winding was investigated using Colburn-j factor (j), Darcy friction factor (f) and JF factor (JF), respectively. Finally, the optimal oil inlet/outlet position and block washer number were identified and the performance of the winding in the optimized design was further evaluated. The results showed that both the oil inlet/outlet position and block washer number had a significant impact on the disc temperature of the winding. The number of overheated discs was reduced by 50% when the oil inlet was set near the outside wall of the winding, compared with the conventional design in which the oil inlet was set at the middle of the winding. Furthermore, the maximum and mean temperature rises of the winding were reduced and the comprehensive performance was improved by setting the block washers. The optimal design for the winding was that the oil inlet was allocated near the outside wall of the winding, the number of the block washer was 3 and the oil mass flow rate was 2m_o. In this optimal design, the maximum and mean temperature rises of the winding were reduced by 56.1% and 63.9%, respectively, and the comprehensive performance was improved by 145%, compared with the conventional design.