Abstract: The energy dissipated during the movement of a fluid viscous damper is stored in the silicone oil as heat, leading to an increase in temperature. This self-heating effect further influences the viscosity of the silicone oil, altering the mechanical performance of the damper. This study aims to reveal the mechanism and the impact of the thermo-mechanical effect on the damping performance of the nonlinear viscous damper. The self-heating mode of the nonlinear viscous damper under harmonic loads was theoretically derived and validated through fluid modeling. Cyclic tests were conducted to reveal the thermo-mechanical coupling effect of the viscous damper. A calculation approach was proposed based on the heat-energy equivalence principle, aiming to obtain the equivalent damping coefficient of the damper throughout the whole loading period of harmonic and seismic excitations. Based on a single-degree-of-freedom system, time history analyses were performed to investigate the performance degradation of the viscous damper under earthquakes with varying intensities and durations. The research results indicate that the damper behaves approximately like an adiabatic container, and that the temperature rise during its operation is linearly and positively correlated with its energy dissipation. The calculation method proposed for the equivalent degradation damping coefficient can characterize the degree of performance degradation of the damper, revealing a negative exponential function relationship between its damping performance and temperature. The temperature rise and degree of performance degradation of the damper demonstrate a super-linear relationship with the intensity and duration of the earthquake. It is crucial to consider the thermo-mechanical coupling effect under long-duration and high-intensity loads.