It is essential to measure the stitch length of weft knitted fabric in a static environment for economy and improving productivity. The image inputted was dealt with the digital image processing techniques: the paper carded out the transformation of grey scale; then removed impurities with media filtering; the characteristic of the stitch structure was abstracted by using the Fourier transform technique. To obtain the stitch length more accurately, we should establish a geometric model of the stitch based on the digital image processing, which provides a way to transform the stitch length in the two-dimension space into a three-dimension space. This study provides a new method for measuring the stitch length without damaging the fabric.