Abstract: In order to improve the accuracy of the existing transformation ratio model of the hydraulic transformer, a mathematical model is established by introducing the friction coefficient, the viscous drag friction coefficient and the laminar leakage coefficient. As shown from calculation and experimental results, the transformation ratio is greatly influenced by the load flow rate and should be modeled as the function of both the control angle and the load flow rate. And at the constant control angle, transformation ratio decreases monotonously along with the load flow rate increasing. And at the constant load flow rate, transformation ratio has a finite maximal value at a certain control angle. This maximal value and its corresponding maximal effective control angle decrease with the load flow rate increasing.