Objective To explore the universal behavior or law of viscoelastic properties of biological soft tissues. Methods According to mechanical structure of biological soft tissues, four-element viscoelastic structure models were built, based on the varied configurations of two linear springs and two dashpots. The differential equations for motion of the viscoelastic models were derived through their geometry, and then were applied to analyze the stress relaxation, creep behavior, stress relaxation time and creep time of the four-element models. Results All the possible four-element viscoelastic models had universal constitutive relation, stress relaxation and creep function. The comparison between the model prediction and the experimental data from various tissues including the aortic valves, ligaments and cerebral artery illustrated that the mechanical behaviors of biological soft tissues could be adequately characterized by the four-element viscoelastic models. Two characteristic time τ1 and τ2 had a significant effect on stress relaxation of biological soft tissues, and the ‘fast’ relaxation time τ1 had a significant effect on the time required for the stress to reach the equilibrium state, while the ‘slow’ relaxation time τ2 didn’t obviously influence the relaxation rate, but had obvious influences on the state of stress relaxation. Conclusions The time-dependent behaviour of biological soft tissues can be characterized by two characteristic time scales, known as ‘fast’ and ‘slow’ time. The stress-strain relationship, stress relaxation and creep function of biological soft tissues with two characteristic times have the same mathematical form, which is independent of the configuration and arrangement of the selected spring and dashpots. However, in order to ensure the rationality of mechanical parameters of the model, suitable models should be selected for different biological soft tissues.