As the prime after-treatment device for diesel particulate matter (PM) emission control, Diesel Particulate Filter (DPF) has been widely used for its high capture efficiency. In order to study the particulate collection capacity of DPF from microcosmic point of view, a wall flow DPF channel model has been built to observe the movement and deposition process in channels of particles with different diameters using numerical simulation method. Through solving the continuity and the Navier-Stokes equations, the flow pattern and velocity inside inlet and outlet channel have been examined. Results show that there is a maximum axial velocity around midway in inlet channel while the axial velocity in outlet channel increases all the way from the plug surface to the exit. As for the study of particle motion and deposition process, this paper uses Euler-Lagrange approach to depict particle trajectories and finds that Brownian force has a crucial effect on fine particles with random motion trajectories which enlarge opportunities being trapped by the porous wall and make contribution to particle collection. With the increase in particle size, the directionality of particle motion enhances under the domination of inertia, which results in that the deposition area of particles gradually moves to the end of the inlet channel. In addition, the trajectories and deposition distributions of particles with different sizes at variable space velocities were investigated in this paper. The understanding this paper get can offer help on further research in optimizing DPF efficiency and lay a foundation for microcosmic study of DPF regeneration process.