Abstract: The accurate simulation of the initial distribution of the fast ions from neutral beam injection (NBI) in Tokamak plasma is the primary task of using large scale numerical simulation method to study the NBI fast ion related physics problems. A simplified physical model for NBI Tokamak plasma was established. A NBI application code was developed independently by using Monte Carlo method. Two typical cases of tangential injection and vertical injection in ASDEX Upgrade (AUG) Tokamak were chosen as computational examples. The initial 3D spatial positions of the ionized NBI particles were simulated respectively, and the initial radial distribution, poloidal angle distribution, toroidal angle distribution and pitch angle distribution of these fast ions were statistically obtained. These calculation results are in agreement with the values from the relevant international literature.