A low-cost and high-efficiency method for fabricating variable line-space gratings is reported. Specifically, oxygen plasma was used to process the stretched wedge polydimethylsiloxane substrate. After slowly releasing the stress, a gradient grating with a continuous change in line spacing was formed via self-assembly. The period of the self-assembly grating was 2.96 to 3.25  μm, from thick to thin, when the sample was stretched transversely. In longitudinal stretching, the period was 3.71 to 5.20  μm from thick to thin. The period variation range of the gratings obtained by the two types of stretching was 15% and 45%, respectively. Diffraction tests of the two samples showed that the diffraction spots were evident. The diffraction angles varied in the range of 11.31 deg to 12.41 deg and 7.12 deg to 9.93 deg. The gratings prepared using this method can compensate for the shortcomings of the traditional process and have a wide range of application prospects.