Frequent and long-term operation at off-design modes of thermal power units is becoming a common phenomenon worldwide due to penetration of intermittent renewable power sources. The high-pressure control valve of a steam turbine is a key component for adjusting its power output, and its characteristics are of great significance for quantifying performances of a thermal cycle. However, it still remains a challenge to model and analyse the characteristics of a high-pressure control valve due to lack of post-valve measurement points. In this paper, we propose a hybrid modelling approach, based on first-principle mechanism and operational data, for characteristics of a high-pressure control valve operating at off-design modes. The flow characteristics of a high-pressure control valve are summarized as a function of flow coefficients. Results show that a characteristic function of a high-pressure control valve can be well used for modelling performances of a control stage operated at variable working conditions. This provides opportunities for improving efficiency of a thermal unit by better adjustment of the control stage of a steam turbine according to quantitative representation of its characteristics.