This invention provides a simulation method for particle beam irradiation, which comprises: a sub-beam approximation step of approximating a particle beam with combination of a plurality of sub-beams having Gaussian distribution respectively； and a sub-beam dose distribution calculation step of simulating the proceeding state of the plurality of sub-beams after being deflected by a scanning apparatus, to thereby calculates respective sub-beam dose distribution formed by each sub-beams inside the patient, then accumulates each calculated sub-beam dose distribution to obtain the dose distribution inside the patient formed by the particle beam.本發明之粒子線照射的模擬方法係具備：副射束近似步驟，係以分別具有高斯分佈之複數個副射束的集合來近似粒子線；以及副射束線量分佈演算步驟，係模擬複數個副射束之各個副射束被藉由掃描裝置偏向而行進之狀態，藉此演算各個副射束形成於患者內部之各個副射束線量分佈，並藉由累加所演算之各個副射束線量分佈，而求出粒子線形成於患者內部之線量分佈。
