Abstract: This study proposes a novel magnetic-modulated proton therapy, explores tumor and organ doses as functions of the magnetic modulation method, and investigates the application of the magnetic-modulated therapy in curing organ-surrounded tumors. Based on Geant4 Monte Carlo code, we constructed an ideal organ-surrounded structure and an abdominal structure comprising a pancreatic tumor. Bragg peak positions of proton beams were modulated through altering the strengths and directions of the magnetic fields inside the two geometric structures. Following the magnetic-deflected transportation tracks, the proton beams were modulated to bypass the vital organs and irradiate the tumor. For the ideal organ-surrounding configuration, the tumor can be sufficiently covered by 95% relative doses, and the vital organ volume receiving proton irradiations can be controlled to nearly zero. For the abdominal structure comprising the pancreatic tumor, the proton beams modulated by magnetic fields can bypass the spinal cord and left kidney and irradiate the pancreatic tumor. Sufficient coverage of 95% relative doses can be achieved through the magnetic-modulated proton therapy method.