Dynamic exchange of water has been shown to effectively trigger thermally responsive shape memory effect (SME) in shape memory polymer (SMP) at room temperature. However, the working mechanism of cross-relaxation effects, which are generated simultaneously from two stimulus fields of water and temperature, has not been understood. In this study, a ‘cross-relaxation effect’ model was formulated for describing water-induced relaxation, thermomechanical behavior and strain recovery of amorphous SMPs. Based on Maxwell’s principle, the ‘cross-relaxation effect’ model was firstly proposed to quantitatively separate the effect of dynamic exchange of water on the thermochemical SME in SMP. Then the water-induced relaxation and strain recovery have been simulated and predicted using this model. Finally, experimental data were used to verify the proposed model, and the numerical results were found to fit well with the experimental ones. This ‘cross-relaxation effect’ model is expected to provide an effective tool for precise design and control of water-induced SME and shape memory behaviors by means of the dynamic exchange of water.