The essential purpose of this research is to exploit and innovate a yarn-level relaxation deformation method for weft-knitted stitches. Each loop formed by knitting yarns was taken as a mass respectively. Spring-mass model at the yarn level was established for several knitting rows and spatial coordinates were assigned to each mass. Based on the intermeshed relationship of loops in the longitudinal direction, the spring-mass model at the yarn level was transformed into a two-layer spring-mass model at the stitch level, with correspondence between the masses of the two models. Each mass was subjected to eight spring forces from different directions. The resultant force was calculated based on the yarn-level mass model, and the mass displacement was solved by the Verlet numerical integration method. According to the coordinates of masses, the geometric models of different knitting types were established, and a matrix operation method for the spatial rotation and translation of control points was proposed, which could quickly calculate the coordinates and accurately simulate the shape of loops. Finally, we simulated the relaxation deformation of five typical weft-knitted stitches. The results show that this method can accurately and effectively simulate the overall deformation effect of the knitted stitches, and the simulation results are similar to the actual results. The analysis results of the pattern size and the simulation efficiency indicate that the larger the pattern, the more loops there are, the overall simulation time is longer and the running memory occupied is larger.