<p>The radioisotope actinium-225 (²²⁵Ac) has been successfully used for targeted alpha therapy in preclinical and clinical applications because of its excellent nuclear characteristics. Medium- and high-energy proton-spallation reactions on thorium are the most important methods for producing ²²⁵Ac. This study examines the possibility of producing ²²⁵Ac by irradiating thorium oxide with medium-energy protons. Thorium-oxide sheets were irradiated with 40-, 50-, 60-, 70-, and 80-MeV protons on the Associated Proton-beam Experiment Platform (APEP) of the China Spallation Neutron Source (CSNS). The cross sections for the formation of ²²⁵Ac were measured using the activation method and offline gamma-ray spectrometric technique. The experimental results were compared with the existing data from EXFOR as well as the theoretical data from the TALYS-based evaluated nuclear-data library. Based on the experimental cross section and theoretical calculations, the production yield of ²²⁵Ac in the irradiated thorium targets was examined. The results showed that APEP can produce sufficient quantities of ²²⁵Ac for purification and clinical therapy. This work is the first measurement of proton-induced nuclear-reaction cross sections at the CSNS APEP.</p>