Aim To study the construction of carotid vulnerable plaque risk model and its relationship with cognitive impairment and prognosis in patients with acute cerebral infarction (ACI). Methods 180 patients with carotid vulnerable plaques in Lianyungang First People's Hospital from July 2018 to July 2020 were selected as observation group, and another 180 patients with unstable plaques who underwent physical examination were selected as control group. The unstable plaque integral ratio, the maximum length of plaques, the maximum thickness of plaques, ulcer plaques, area stenosis, peak flow velocity at stenosis and resistance index were compared to study the construction of carotid vulnerable plaque risk model and its relationship with cognitive impairment and prognosis in patients with ACI. Results There were significant differences between the two groups in unstable plaque integral ratio, maximum plaque length, maximum plaque thickness, ulcer plaque, area stenosis, peak flow velocity at stenosis and resistance index (P＜0.05). Multivariate analysis showed that high unstable plaque integral ratio, maximum plaque length, maximum plaque thickness, ulcer plaque, peak flow velocity at area stenosis and resistance index were the risk factors for carotid vulnerable plaque. The incidence of cognitive impairment (χ²＝11.432, P＝0.001) and poor prognosis (χ²＝14.362, P＝0.000) in the observation group was significantly higher than those in the control group. Correlation analysis showed that carotid vulnerable plaques in ACI patients were significantly correlated with cognitive impairment and prognosis. Conclusions Carotid vulnerable plaque is significantly correlated with cognitive impairment and prognosis in patients with ACI. High unstable plaque integral ratio, maximum plaque length, maximum plaque thickness, ulcer plaque, area stenosis, peak flow velocity at stenosis and resistance index are all risk factors for carotid vulnerable plaque, it is suggested that clinical intervention should be carried out in time for such patients.