Objectives Effective diagnosis and treatment of abdominal masses in an elderly Pekingese come for medical consultation was conducted to provide reference for the research and diagnosis of similar cases. Methods The clinical examination, routine and biochemistry examination of blood, and histopathological methods were used to diagnose the elderly Pekingese. Comprehensive treatments were carried out based on the results of diagnosis. Results The patient Pekingese was diagnosed with breast tumor. The breast tumor was successfully removed through surgery, and the result of pathological diagnosis showed that the breast tumor was high-grade ductal carcinoma in situ. Systematic treatment was performed after surgery, and recurrence occurred 3 months later. The result of DR imaging showed that it was cancer metastasis. Conclusions It is important to observe the physical condition of dogs regularly and seek medical consultation promptly at a reputable animal hospital if any abnormalities are detected. Early detection and treatment of breast tumors are essential for achieving good effect of treatment with a higher rate of recovery.