Objective To investigate the diagnostic value of serum soluble CD40 ligand (sCD40L) and matrix metalloproteinase-9 (MMP-9) in carotid plaque instability in the elderly patients with cerebral ischemic stroke (CIS). Methods A retrospective analysis was made of the clinical data of 266 elderly (≥60 years old) CIS patients (CIS group) admitted to Yantai Penglai Hospital of Traditional Chinese Medicine from August 2021 to December 2022 and 234 healthy volunteers who underwent physical examination in the same hospital during the same period (control group). The two groups were compared in serum sCD40L and MMP-9 levels to analyze their value in the early diagnosis of CIS. According to the carotid plaque status, the CIS group was divided into a plaque stable group (n=106) and a plaque unstable group (n=160). The factors affecting plaque instability in the elderly CIS patients were analyzed, and the diagnostic value of serum sCD40L and MMP-9 levels in the carotid plaque instability among them was investigated. SPSS statistics 20.0 was used to analyze the data. Depending on the data type, t test orχ² test was used for comparison between groups. Multivariate logistic regression was used to analyze the factors affecting carotid plaque instability in the elderly CIS patients, and the ROC curve was used to analyze the diagnostic efficacy of serological indicators in carotid plaque instability in them. Results The serum levels of sCD40L and MMP-9 in the CIS group were significantly higher than those in control group with statistically significant difference (P<0.05). All patients in the control group were plaque free, and in the CIS group, 106 patients had stable plaques and 160 patients unstable plaques. In comparison, the plaque unstable group and the plaque stable group showed statistically significant differences in the levels of total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), sCD40L, MMP-9, fibrinogen (FIB) and white blood cell count (WBC) (t=13.008,9.511,14.514,20.972,13.252,16.344,10.232; P<0.05). Multivariate logistic regression analysis showed that TC (OR=1.758,95%CI 1.083-2.852), LDL-C (OR=2.275,95%CI 1.045-4.954), sCD40L (OR=1.956, 95%CI 1.112-3.440), and MMP-9 (OR=1.846,95%CI 1.151-2.961) were independent risk factors of carotid plaque instability in CIS patients (P<0.05). The area under the curve (AUC) of serum sCD40L and MMP-9 for carotid artery unstable plaque diagnosis in elderly CIA patients was 0.967 and 0.939, respectively. Conclusion TC, LDL-C, sCD40L and MMP-9 are independent risk factors for carotid plaque instability in CIS patients, and clinical monitoring of these indicators should be strengthened. In addition, the monitoring of serum sCD40L and MMP-9 levels can provide an objective basis for the early diagnosis of CIA and the assessment of carotid plaque instability in CIA patients.