This study focuses on the uniqueness of rate-dependency, creep and stress relaxation behaviors of Shanghai soft clay based on different time-dependent tests. A one-dimensional elasto-viscoplastic model is briefly introduced based on the rate-dependency of preconsolidation pressure. The relationship between the ratedependency and the creep is firstly described. Then the strain-rate based formulation is extended to derive the evolution of vertical stress during relaxation with an analytical solution.A stress relaxation coefficient is defined as the slope of ln(σi v) versus ln(t). Based on this, the relationship between the rate-dependency coefficient and the stress relaxation coefficient is derived by solving the differential of ln(σ i v) versus ln(t). The uniqueness between the rate-dependency, the secondary compression and the stress relaxation is validated by comparing the three experimental and estimated time-dependent coefficients of Shanghai soft clay.