Facing the severe situation of air pollution, more stringent regulations of pollutant emissions are being promulgated in China, which calls for more accurate and reliable monitoring of particulate matter (PM) emission in coal-fired power plants. In this work we study the sampling loss of CEMS under different conditions by numerically solving particle transport equation in the sampling line. Featuring with a high Reynolds number, the particle deposition loss is severer than conventional laminar sampling and increases with Reynolds number when the plant load changes. The temperature difference between the hot sampling gas and pipe wall has a great effect on the sampling loss of PM10. A small temperature difference of 2 K, which is very likely to exist even with a thick thermal insulation, will increase the deposition velocity of PM1-2.5 by ten times. The surface roughness, either from pipe itself or deposited particles, also enhances the deposition loss by partly shifting the capture boundary to a higher diffusivity region. Combined all the possible factors, the loss ratio of 10 µm particles could reach 69% after 0.2 s and 95% after 0.5 s. The loss ratios of 2.5 and 1 µm particles are much lower, but also reach 4.1% and 7.9% respectively after 1 s, which cannot be neglected when a high accuracy monitoring is needed.