The uniaxial and biaxial stress relaxation tests of PVC(polyvinyl chloride) membrane were carried out. The uniaxial stress relaxation was simulated by Prony series. The fitting equations of uniaxial and biaxial stress relaxation were established. The results show that the initial stresses have little effect on the uniaxial stress relaxation in the warp and weft direction; under different initial stress ratio of warp direction to weft direction, the biaxial stress relaxation values in warp and weft direction change a little; the biaxial stress relaxation values are larger than the uniaxial stress relaxation values in warp direction, whereas results in weft direction are opposite. It is found that the uniaxial stress relaxation could be described by the Prony series. Through introducing adjusting functions into fitting equations of uniaxial stress relaxation, the biaxial stress relaxation could be well simulated.