Considering the high in-situ stress of the marble in the underground powerhouse of Jinping Ⅰ Hydropower Station, the intact marble samples were prepared into fractured specimens through unloading triaxial tests in the laboratory, and then triaxial creep tests were conducted on the unloading fractured marble specimens to reveal the axial and lateral creep properties and confining pressure effect. Under the constant axial stress, the lateral deformation of unloading fractured marble exhibits more significant creep characteristics than axial deformation. Besides, the effect of confining pressure on lateral creep deformation is more obvious. In terms of the nonlinear characteristics of creep and the obvious lateral creep of unloading fractured marble, the Newton dashpot in the generalized Kelvin model was replaced by a Koeller fractional dashpot. Meanwhile, considering the unsteady characteristics of model parameters under the influence of stress and time, and combing with the equation of the expansion ratio-time relationship, an unsteady fractional model that can comprehensively describe the lateral creep characteristics of unloaded fractured marble was established. The test results verified that the model can better describe the nonlinear creep characteristics of the axial and lateral deformation, which is critical for studying lateral deformation of surrounding rock under high stress and strong unloading conditions.