Abstract 4D printing is a novel concept and multidiscipline research area which integrates advanced manufacturing, material science and mechanics. As an active material with high stiffness and rapid response speed, shape memory polymer (SMP) is widely used in 4D printing structures. Accordingly, a simple but effective theoretical model becomes highly desirable to better assist the 4D printing design. In this paper, a phenomenological model is developed in which the concept of phase evolution is used to describe the glass transition behavior of SMP. Explicit iterative format has been derived where the stress–strain behaviors of phase generation and phase vanishment are distinguished. The model is further employed to predict the shape memory behavior of pure SMP and active bending behavior of SMP-elastomer bilayer composites. With its inherent conciseness and generality, the model is promising to be applicable in modelling other soft active materials with phase evolution behaviors.