Objective: To investigate the relationship between ambulatory arterial stiffness index, circadian rhythm and TCM syndrome differentiation in patients with essential hypertension. Methods: Ambulatory blood pressure of the patients with essential hypertension was monitored from March 2009 to March 2014 in our hospital. The data of ambulatory arterial stiffness index (AASI), Symmetric ambulatory arterial stiffness index (Sym-AASI) and blood pressure circadian rhythm (BPCR) was collected. The association between AASI, Sym-AASI, BPCR and TCM syndrome differentiation were analyzed. Results: The levels of AASI and SAASI in hypertension patients with excessive accumulation of phlegm-dampness syndrome were significant increased than others, which indicated the poor elasticity of vessel and the degree of hardening is more serious. The nocturnal blood pressure fall rate in patients with hyperactivity of liver-fire syndrome was lower than others, meaning abnormal circadian blood pressure. Conclusion: The ambulatory blood pressure monitoring index is associated with TCM syndrome differentiation in patients with essential hypertension, which plays certain instructive role in the treatment of various types of TCM syndrome. © 2014 IEEE.