Abstract: The influence of loading and unloading history on creep deformation suggests that the cyclic stress environment in underground engineering e.g.: excavation, backfilling, and unloading) can significantly impact the long-term stability of salt rock. To further investigate this effect, this study systematically examined the impact of different loading and unloading conditions on the creep behavior of salt rock through stepped loading and unloading creep tests conducted under identical stress levels. A new creep constitutive model was developed by introducing a state variable that characterizes the degree of rock hardening, effectively accounting for the effects of loading and unloading on salt rock deformation. The results indicated that creep strain under stepped loading and unloading exhibited significant differences due to the influence of loading and unloading history. The stepped loading test led to a gradual increase in creep strain, whereas the stepped unloading test resulted in negative creep. This behavior occurred because, during stepped loading, the salt rock underwent multiple incremental stress stages. As the stress level increased, continuous evolution of the internal microstructure facilitated the progression of creep deformation. In contrast, during stepped unloading, internal structural adjustments were primarily driven by residual internal stress. Structural hardening induced high-stress levels continued to dominate in subsequent low-stress stages, resulting in strain, reversal—manifested as negative creep. The constitutive model based on a state variable accurately predicted the creep deformation of salt rock under both loading and unloading conditions. Model fitting results showed excellent agreement with experimental data, demonstrating its effectiveness in characterizing and predicting creep behavior and historical effects. Additionally, salt rock deformation can be categorized into time-dependent and time-independent components. Time-dependent deformation was influenced by both stress and time, resulting in varying deformation under different durations. Time-independent deformation included elastic and plastic components during loading. To clearly distinguish creep from plastic deformation, the time-independent plastic deformation was termed “loading plastic deformation” in this study. This deformation was closely related to the stress state before and after loading, but was independent of the loading path, unloading path, rate, and duration. Sensitivity analysis of the model parameters revealed that parameters a and b affected creep behavior by influencing constant and variable creep strain, respectively. Parameters k, m, and c influenced creep behavior through the state variable. Parameter n characterized the stress sensitivity of creep deformation and affected the deformation behavior at different creep stages.