Yarn supply tension is a key factor which has influences on the quality of knitted fabrics. This paper introduces a method to detect the yarn tension during knitting action. A 3-rollor tension sensor was adopted and a sensory system was assembled on a flat knitting machine. A single chip microcomputer and a specially designed circuit were used for the signal capturing and transmission. Yarn tension changes during the knitting process was recorded and displayed by a PC. In order to verify the validity, several experiments were arranged. The results indicate that the yarn tension can be detected, recorded and displayed. After analyzing the tension curves, it is founded that the knitting action especially the yarn elastic properties and feed spring performance affects the yarn supply tension. We also find this method is practical.