Abstract: A comparison was made between the testing principle of dynamic breaking strength and that of static breaking strength,and quantitative analysis of the test results was conducted via mathematic method.It was found that the dynamic strength of yarns was 20%～30% lower than that of the static.This shows that in evaluating yarn quality index,dynamic strength is more useful in practice. It also pointed out that use of CTT(constant tension transport) in testing dynamic strength of yarns can simulate,with different testing speeds,the yarn in use and can display the strength of weak points of yarn,more truly reflecting the strength state of yarn during production.