Introduction: CAVD is a progressive disease resulting in severe aortic valve stenosis. Recent studies unveiled protective effects of FSTL1 in various cardiovascular disorders. But its role in CAVD is uncertain. This study aims to evaluate the clinical and pathophysiological relevance of FSTL1 in CAVD. Methods: FSTL1 level was measured in blood samples fromCAVD (n = 343) and non-CAVD (n = 318) patients. The diagnosis of CAVD was based on the echocardiography report. ApoE-/- mice and primary aortic valve interstitial cells (VICs) were used as animal and cell models respectively for mechanistic studies. Results: Compared with non-CAVD group, CAVD patients exhibits lower circulating FSTL1 concentrations [Median (IQR) ng/mL: 0.91 (0.52-1.50) versus 0.78 (0.34-1.25), p=0.002]. Multivariate logistic regression indicates that circulating FSTL1 level is independently associated with CAVD (OR: 0.831, 95% CI: 0.755-0.915, p<0.001) after full adjustment. Moreover, there is a non-linear relationship with threshold effect between FSTL1 level and CAVD: Below the turning point (FSTL1= 1.2ng/mL) the predicted probability of CAVD decreases significantly as FSTL1 level increases, but above 1.2ng/mL, the downtrend levels off. In ApoE-/- mice, overexpression of FSTL1 significantly reduces calcified nodule formation and structural remodeling of aortic valves. Finally, rhFSLT1 significantly alleviates calcium deposits, alkaline phosphatase activity, suppresses expression of osteogenic markers (BMP2, RUNX2, and MSX2) and inhibits phosphorylation of ERK1/2 in VICs in the osteogenic medium Conclusion: FSTL1 emerges as an important marker exhibiting inverse clinical association with CAVD and it alleviates the mineralization of murine valve and VICs. Further evaluation and manipulation of FSTL1 might provide a novel potential therapeutic opportunity for CAVD.