The diesel particulate filter (DPF) is an effective device for reducing particulate emissions from diesel engines, while its durability and reliability after long-term use are causes for concern. Usually, particulates are considered to be uniformly deposited in DPF channels to form a cake or end plug, however, recent studies have found that a “middle channel deposit” phenomenon of particulates can form a bridge near the middle of the DPF channel. This phenomenon has serious adverse effects on the durability and reliability of the DPF, including abnormally increased pressure drop and frequent regeneration. Since the width of the DPF channel is only about 1-2 millimeters, conventional methods cannot observe the particulate deposition process inside the DPF. In order to deeply understand the mechanism for the formation of the bridge phenomenon, this paper establishes a verified three-dimensional model of the DPF channel, including two 1/4 inlet channels and two 1/4 outlet channels, so as to observe the particulate deposition process. The effects of exhaust parameters (including mass flow, exhaust temperature, and particulate size) and structural parameters (including permeability, wall thickness, and cell density) on particulate deposition characteristics in the channels were investigated. It is found that the particulate deposition position is closer to inlet at small exhaust flow and low exhaust temperature conditions. Reducing the wall thickness and cell density can also increase the number of particulates deposited upstream and the middle of inlet channel. The results clarify the causes of “middle channel deposit” of particulates, which will help to understand the particulate deposition process in millimeter DPF channels, so as to provide ideas for the optimal design of DPF, as well as the improvement of durability and reliability.