To analyze the stress relaxation behaviors of PET film in ionic solutions, 5-element mechanical model and its constitutive equations were proposed. And the constitutive equations having two different relaxation mechanisms were used to examine the validity of constitutive equations in 5-element mechanical model. As the radii of ions were decreased, the relaxation stresses were increased. But the relaxation stresses were increased with the increase of draw ratio, ionic strength and ionic value. Besides the relaxation stress, the parameters of constitutive equations in relaxation process were investigated.