Aiming at the annually measured continuous growth phenomenon of the dam body deformation towards the downstream direction for Jinping-I Arch Dam when the reservoir keeps stable at the normal water level elevation of 1 880 m, back analysis was conducted for the viscoelastic working behavior of the dam based on the measured displacement data and the concrete creep data. The results show that the dam body is in a working state of viscoelastic at present, and the measured short-term trend deformation is jointly caused by the hysteretic hydraulic deformation and the ambient temperature drop effect. Good linear relationships exist between the viscoelastic mechanical parameters of the dam concrete. The elastic modulus of the dam concrete through back analysis according to the elastic hydraulic component is actually an equivalent value of the instantaneous and delayed elastic modulus. For Jinping-I Arch Dam, this inversion value is 35. 5 GPa, which is close to the earlier back-analysis value and experimental value. The instantaneous elastic modulus, two delayed elastic moduli and two viscosity coefficients in the generalized Kelvin model are 47. 3 GPa, 189. 4 GPa, 132. 3 GPa, 383. 8 GPa·d and 20 574. 6 GPa·d, respectively. To establish deformation monitoring models for Jinping-I Arch Dam, it is recommended that a new viscoelastic hysteretic hydraulic component should be added to the traditional hydraulic, seasonal and time(HST)model.