Based on the in-phase thermomechanical fatigue experiments under stress control, the damage mechanism of in-phase thermomechanical fatigue of DD6 superalloy is investigated. Based on the continuous damage mechanics, a lifetime prediction model reflecting the coupling of creep damage and fatigue damage is established, and the damage parameters in the model are obtained through using the experiment data of creep and fatigue. Furthermore, the experiments of in-phase thermomechanical fatigue with dwells of DD6 superalloy are conducted in this paper, and the experiment lifetimes are within a factor of 2.3 of the prediction lifetimes based on the coupling damage, which indicates that the coupling damage based lifetime prediction model developed in this paper could accurately predict the in-phase thermomechanical fatigue lifetimes of DD6 superalloy and provide a basis for the lifetime prediction of DD6 superalloy structure in engineering applications.