Objective To study the value of 18F-FDG PET CT combined with MRI in the evaluation of carotid atherosclerotic plaque. Methods A total of eighty patients with carotid atherosclerotic plaques who were admitted to the hospital during the period from February 2016 to February 2018 were selected as study subjects. All patients were examined with 18F-FDG PET CT and MRI within 2 weeks. The region of interest (ROI) was drawn around the plaque to obtain the standardized uptake value (SUV), and the target-to-background ratio (TBR) was calculated. The integrity and thickness of fibrous cap were observed, and plaques were divide into non-fibrous cap, thick fibrous cap, thin fibrous cap and ruptured fibrous cap. They were grouped according to the composition of plaques, including collagen group, calcification group, lipid group and bleeding group. Results Of the 80 patients with carotid atherosclerotic plaques, 7 patients with poor image quality were excluded. A total of 1930 layers were scanned. Most of the plaques were located around the bifurcation of the carotid artery. There are 1065 layers without fibrous cap, 547 layers with thick fibrous caps, 202 layers with thin fibrous caps, and the other 116 layers with ruptured fibrous caps. Grouping according to the composition of plaques showed that there were 236 layers of collagen, 796 layers of calcification, 730 layers of lipidsand 168 layers of bleeding. The TBR in the fibrous cap rupture group was significantly higher than that in the other three groups (P<0.05). The TBR in the thin fibrous cap group was higher than the thick fibrous cap group, and higher than the nonfibrous cap group (P<0.05). The TBR in the bleeding group was significantly higher than that in the lipid group, calcification group or collagen group (P<0.05). The TBR in the lipid group was significantly higher than that in the calcification group or the collagen group (P<0.05). Conclusion MRI can evaluate the fibrous cap and composition of plaques qualitatively. 18F-FDG PET CT can reflect the degree of inflammatory activity of the plaque. The combined use of the two methods plays an important role in evaluating the stability of carotid atherosclerotic plaques.