Cooperative principle in shape memory polymers (SMPs) result in their effective communication with the external thermochemical stimuli and then the SMPs act cooperatively to generate stimulus-responsive shape memory effect (SME). Their SME can be triggered by chemical stimuli and the corresponding thermochemically responsive behaviors are often explained by the reversible rearrangement of macromolecule segments due to changes of their configurational entropy. In this paper, we propose a new cooperative model to describe the multiple glass transitions and strain relaxations of an SMP in response to the water by means of selective and chemical plasticization effect on the partial segments in a macromolecule. Based on this model, the plasticization effect on the glass transition temperature, relaxation time and recovery strain of the SMP have been systematically investigated. Finally, the constitutive model for thermochemically responsive SMPs with two viscoelastic transitions and multiple SMEs was established to describe their overall mechanical responses, and a good agreement between the numerical and experimental results has been achieved.