Objective To investigate the value of quantitative parameters of carotid ultrasound combined with MRI in the diagnosis of acute cerebral infarction (ACI). Methods The cases were from 106 patients with ACI from December 2016 to September 2018. Carotid ultrasound was performed to examine carotid stenosis, carotid intima-media thickness (IMT), internal carotid peak systolic velocity (PSV), end diastolic velocity (EDV) and plaques in ACI patients. MRI scan was performed to examine carotid stenosis, and the nature and distribution of carotid plaques. Results The IMT in moderate to severe group was significantly higher than that in mild group, while PSV and EDV were significantly lower than those in mild group(P<0.05). The ROC curve analysis showed that the area under the curve of IMT with PSV and EDV for the diagnosis of carotid stenosis degree in ACI patients was 0.821. There were 102 cases with positive carotid atherosclerotic plaques detected by MRI, the detection rate was 96.23% (102/106). 79 of plaques were located in the common carotid bifurcation (50.00%), and there were 107 (67.72%) vulnerable plaques. The plaque lipid component showed equal signal in 2D-TOF image, equal or slightly lower signal on T1WI, slightly lower signal on T2WI. The complete fiber cap showed banded low signal on 2D-TOF, equal or higher signal on T1WI and T2WI. There were 86 cases with positive carotid atherosclerotic plaques detected by ultrasound, the detection rate was 81.13% (86/106). The overall detection rate of ultrasound combined with MRI (98.11%) for carotid stenosis degree in ACI patients was significantly higher than that of either of the two examination methods (ultrasound 70.75%, MRI 84.91%) (P<0.05). Conclusion The application of quantitative parameters of carotid ultrasound combined with MRI in the diagnosis of ACI can help accurately qualitative and locating diagnosis for carotid plaques, which significantly improve the detection rate of carotid stenosis.