Prostate cancer is the second most common cancer in men in the world. Proton therapy is one of the effective methods in the treatment of prostate cancer. In this study, using the MCNPX simulation code and using the ICRP110 adult male reference phantom, the risk of secondary cancer with proton therapy was studied. In this study, an active beamline based on a synchrotron accelerator in which the proton beam energy is dynamically changed was considered. Then, according to the calculations performed in this study, the appropriate direction of radiation was obtained. Using Monte Carlo simulation and mathematical calculations in MATLAB software, the appropriate energy range was calculated to create a uniform dose distribution in the desired tumor volume. Then, using a Monte Carlo simulation code, the amount of energy and absorbed dose was calculated using Tally  in the tumor, prostate, and healthy and sensitive tissues per one Gy proton therapy dose at the center of the SOBP Bragg peak. Then, the proton equivalent dose, neutron equivalent dose, and secondary photon equivalent dose in different tissues were calculated. Calculations showed that the equivalent dose decreases with distance from the tumor center. The secondary dose reached in different tissues and the risk of secondary cancer per one Gray therapeutic dose were also calculated. Studies showed that the risk of secondary cancer in tissues distant from the prostate such as the thyroid, lung and brain and   10^-9 for tissues close to the tumor such as the bladder and testicles was about 10^-5,  10^-6  and  10^-4 for the prostate.