Accurate measurement of ankle-brachial index（ABI） has important significance in clinical medical diagnosis.In the measurement of ABI

accurate measurement of upper and lower limb systolic blood pressure is the key.When the cuff pressure blocks vascular blood flow

the cuff begins to slow deflation.Pulse backing to fluctuations of the first pulse wave corresponding to the cuff pressure is equal to the systolic pressure.Because of in the cuff pressure

the blocking of the lower limb vascular blood flow are not completely

so it is difficult to measure arterial systolic pressure of the lower limb accurately.PCA algorithm is used to separate the hybrid pulse wave

and mathematical morphology gradient theory is used to process the separated pulse wave.The experiment indicates that the application based on mathematical morphology gradient theory can successfully find the starting point of the pulse wave backing to fluctuations

after being separated

therefore this method can measure ABI accurately.