The relationship between equivalent time and rheological velocity is studied in the frame of viscous continuum theory. If rheological velocity is only determined by stress and strain

the rheological velocity in the simple loading history is equal to that in the complex loading history when they have the same stress and the same strain. Then the rheological velocity re-calculated in the simple loading history can be used to establish the model in the complex loading history. The lasting time used to re-calculate rheological velocity in the simple loading history is called the equivalent time

and the stress-strain relationship at the same equivalent time in the set of simple loading histories is called the equivalent time line. According to the above idea for formulating model

the relationship between equivalent time and rheological velocity is established for clay with nonparallel equivalent time lines

and a nonparallel equivalent-time-lines rheological model for clay is achieved. This model can be degenerated into Yin-Grahams 1-D rheological model when equivalent-time-lines are parallel to each other. The parameters of model are determined by trial data and the model is further verified to be valid from laboratory test for clay.