Objective To analyze the clinical value of magnetic resonance imaging (MRI) and 18 fluorodeoxyglucose positron emission computed tomography (18FDG PET-CT) in precise radiotherapy of patients with oropharyngeal cancer after operation. Methods The MRI and 18FDG PET-CT data of 49 patients with oropharyngeal cancer who underwent precise radiotherapy in the hospital from August 2014 to July 2017 were analyzed retrospectively. The value of different examination methods in the diagnosis of residual lesions and positive lymph nodes was compared. The SUVmax and ADC values were compared before and after precise radiotherapy, and between patients with residual lesions and different therapeutic effects to clarify the clinical value of MRI and 18FDG PET-CT in precise radiotherapy ofpatients with oropharyngeal cancer after operation. Results ①The accuracy rate, specificity, sensitivity, positive predictive value and negative predictive value of 18FDG PET-CT in the diagnosis of residual lesions were 94.33%, 94.87%, 92.85%, 86.67% and 97.36%, respectively, and the Kappa value was 0.857. The accuracy rate, specificity, sensitivity, positive predictive value and negative predictive value in the diagnosis of positive lymph nodes were 95.26%,95.58%,92.30%, 80.00%, and98.56%, respectively, and the Kappa value was 0.828. All of them were higher than those of MRI. ②After precise radiotherapy of residual lesions and positive lymph nodes, the SUVmax decreased significantly, with statistically significant difference between groups (P<0.05), but there was no significant difference in SUVmax after 1 month and 3 months of radiotherapy (P>0.05). Conclusion 18FDG PET-CT can be used in precise radiotherapy of patients with oropharyngeal cancer after operation.