To recognize the interrelationship of plastic deformation with strain and time, uniaxial tension relaxation experiments of PVC coated plain weave membrane were executed along the warp and weft direction, while plastic deformation of the specimen were measured in 1 year. Using multiple functions from the product of both the strain difference (strain- critical strain) power function and the time power function, the experimental data was studied on a multiple regression analysis. It was concluded that the model had better fitting results. At the same time, an instantaneous plastic and a relaxed plastic term of the total plastic deformation were analyzed. It was showed that the instantaneous deformation was the main part of the plastic deformation, that the relaxed plastic deformation was the minor part of the plastic deformation, and the relaxed plastic deformation would become smaller and smaller with time extension.