The invention discloses a method for accelerating photon dosage calculation in radiotherapy. Different from the conventional method, the method is characterized in that with the serial calculation of a CPU (Central Processing Unit) for processing complicated logic affairs and the like, the GPU is used for processing two processes of equivalent depth calculation and dosage accumulation in photon dosage calculation, and each process has the characteristics of large data quantity, low data dependency, same data executive program, high parallelism and density and the like. Therefore, more calculating units can be executed in parallel through the GPU so that the operation time is shortened, the calculating efficiency is improved, and when any therapeutic parameters are changed in the optimizing process of a therapeutic schedule, the real-time effect on dosage calculation and dosage distribution display can be basically achieved.