Abstract: The nonlinear ultrasonic response to the co-action of temperature and stress is studied. A mathematic model is first developed to describe the propagating of ultrasonic longitudinal bulk waves in continuously welded rails(CWRs) by incorporating interatomic potentials into the nonlinear wave equation. The functional relationships describing how the nonlinear parameter changes under the influence of temperature, for both cases of free and constrained CWR are obtained. Experiments on CWR under thermal stress changes are carried out. The experimental results are found to be in good agreement with the theoretical expectation of nonlinear parameter variation trend. These results indicate that both of the temperature and stress have obvious influences on the nonlinear ultrasonic longitudinal waves propagating in solids. The nonlinear parameter of the medium is increased with the increase of temperature and decreases with the increase of internal compressive stress, suggesting that it is necessary to compensate the effect of temperature when using nonlinear parameter to detect the thermal stress in CWR.