The invention discloses a double-step Monte Carlo simulation method applied to radioactive ray dose computation. The double-step Monte Carlo simulation method comprises the following steps: step 1, performing Monte Carlo simulation, thereby acquiring valid seed numerical array; step 2, after acquiring the valid seed numerical array in the step 1, by utilizing valid seed numerical values, performing secondary Monte Carlo simulation and computing radioactive ray dose distribution inside the human body at a high speed. The invention provides a double-step Monte Carlo simulation algorithm; simulation of photons acted on a tumor region according to the valid seed numerical values can be performed, and simulation of photons without passing through the tumor region is avoided, so that computational efficiency is improved.