The flue gas of the coal-fired boiler is composed of a great number of fly ash particles. Such particles may easily form ash deposition on the heating surface of the heat exchanger when recovering the wasted heat energy from the flue gas. Here, we study the deposition of gas-side fly ash particles on the heating surface in a three-dimensional rectangular heat transfer channel by a numerical simulation method. First, the critical deposition velocity is calculated using the user-defined function (UDF) and the deposition process of the ash particles are simulated using the discrete phase model (DPM). Then, the rationality of the proposed method is verified by comparing the simulation results with the experimental data. In addition, we investigate the effects of the inlet velocities and particle diameters on the deposition efficiency of each wall of the rectangular channel. The results show that the particle deposition efficiency of different locations of the channel, i.e., the floor, the two sidewalls, and the ceiling, may be varied by the changing the particle diameter and the flow velocity. Moreover, we show that for a large particle, the ceiling and sidewalls, respectively, the first and second lowest particles deposition efficiency, while the floor exhibits the largest particles deposition efficiency. (C) 2019 Elsevier Ltd. All rights reserved.