Thrust density distribution of ion thruster is an important factor that affects the orbit correction and station keeping of the spacecraft. Current empirical models mainly concern themselves with the overall thrust of the ion thruster, yet the thrust density distribution has not been fully understood. Hence it is necessary to investigate the thrust density characteristics of the ion thruster to devise the approach to optimizing the thruster performances. In this study, the thrust density characteristics of the ion thruster is analyzed and discussed by combining the empirical and theoretical methods. An ion thruster utilizes biased grids to extract ions from discharge chamber and accelerate them to high velocities, thereby forming a beam and generating thrust. In this paper, we analyze the working process of the ion thruster. The thrust expression as a function of beam micro-particle parameters is presented. Meanwhile the transport process of the plasma in the beam stream is simulated by the particle in cell-Monte Carlo (PIC-MCC) method for two-grid optics. The motion behavior of ions is modeled by the PIC method, while the collisions of particles are modeled by the MCC method. In the simulation, the particle trajectories are traced and the micro information about ejected charged ions is recorded with respect to singly charged ion, doubly charged ion and charge exchanged (CEX) ion. By analyzing the density and axial velocity of the charged particles in the beam stream, the thrust of the beam from a single grid hole can be calculated, based on which the thrust distribution of the thruster can be inferred by considering the distribution of plasma density at the exit of discharge chamber. Moreover, the above theoretical analysis of the thrust density is tested experimentally. The studies show that the thrust contribution percentages of the singly charged ion, doubly charged ion and CEX ion in the beam current are 84.63%, 15.35%, and 1.82%, respectively. Apparently, the main contributions to the thrust are made by the singly charged ions and doubly charged ions in the beam plasma, while the CEX ions have a trivial effect on the variation of the thrust. The distribution of the thrust density shows good symmetry along the central axis and it levels off after a fast decline in the radial direction. Comparisons of empirical and numerical results with the experimental results show that the empirical results have an error of about 4.1% and the numerical results have an error of about 2.8%. This indicates that the computational accuracy of our numerical model is better than that of the empirical model This work provides a reference for optimizing the thrust density uniformity of an ion thruster.