Objective To explore the evaluated value of magnetic resonance imaging (MRI) and mammography in the preoperative tumor volume and pathological types of breast cancer. Methods 120 cases of patients with breast cancer who were admitted to the hospital from April 2016 to April 2018 were selected. All patients were confirmed to be breast cancer by surgical pathology. All patients were given breast MRI and mammography before operation. The detection rate, tumor volume and pathological types of the two examination methods were compared. Results The detection rate of breast cancer by MRI was higher than that by mammography (P<0.05), and the consistency of tumor volume assessment and pathological results was higher than that of mammography (P<0.05), and the coincidence rates of pathological types of ductal carcinoma in situ, invasive lobular carcinoma and mucinous adenocarcinoma were higher than those by mammography (P<0.05). Conclusion The detection rate of preoperative breast cancer by MRI is higher than that by mammography, and MRI has accurate pathological types, and has high consistency of tumor volume assessment and pathological results.