The widespread adoption of intermittent wind and solar energy sources disrupts the stability of power grids, necessitating frequent load rejection processes for pump-turbine. This jeopardizes the stability of pump-turbine units. To investigate the operating behavior of pump-turbine units during load rejection, this study focuses on a Francis reversible pump-turbine. Employing the results from a one-dimensional characteristic line method as boundary conditions, three-dimensional numerical simulations are conducted. The entropy production theory was employed to quantify energy loss in different components, including direct dissipation, turbulent dissipation, and wall shear stress. And the pressure load curves of runner blades at different spans are extracted. The flow characteristics, entropy production losses, and pressure loads within the passage are analyzed at various moments during the load rejection process. Results demonstrate that when a unit undergoes load rejection, it repeatedly transitions into and out of the S-characteristic region. The flow state within the passage is most unfavorable under reverse pump conditions, with the entropy production loss reaching its maximum value of 339 160 W/K. When the unit's rotational speed increased beyond 1.1nd, the unit's water thrust and torque underwent drastic changes. Moreover, the pressure load undergoes significant variations under runaway conditions, with the pressure load difference among various blades reaching up to 33 000 kPa. These findings provide a scientific basis for ensuring the safe and reliable operation of pump-turbine units during load rejection processes.