This paper analyzes the error between the theoretical calculated and actual values of single inserting yarn strain of multi-axial warp-knitted fabric under the uniaxial stretching process, and the “open M-shaped curved surface” of error was obtained theoretically. The analysis results show that the error increases with the increase of fabric elongation. The error is smaller in the small deformation range (fabric maximum elongation is 5%) and larger in the large deformation range. In addition, the error is also related to the angle between the single inserting yarn axis and the uniaxial tensile direction. The error varies periodically with the angle ( θ ), reaching its maximum when it is close to 90 ° . In order to realize the correction of the existing theoretical calculated formula of single inserting yarn strain, the error formula was fitted by combining the laws of errors. The correction of the single inserting yarn strain was successfully achieved by the fitted error formula. The correction equation can correct the inserting yarn at any angle in the fabric strain range of 0–50%. The correction equation has a larger range of use and more accurate results than the theoretical calculated formula for inserting yarn. Finally, the validation was carried out with bias inserting yarn at ± 30 ° , ± 45 ° , ± 60 ° and ± 75 ° angles and the results were very good. Therefore, the corrected equation can replace the actual value of bias inserting yarn strain.