Based on the visualized filtration system for diesel particulate filter (DPF), the pressure drops under different ash depositions and the effects of ash particle size and ash content on pressure drop were investigated. The experimental results showed that the initial pressure drop increased linearly with deposited ash volume. While the filtration velocity was 0.02 m/s, the ash depositions for the cenospheres of 5 000 mesh, 1 250 mesh and α-Al₂O₃ were less than 4 g/L, 5 g/L and 3 g/L respectively and the pressure drop was obviously lower than that of clean DPF. This proved that ash deposition in DPF could decrease the pressure drop, but it would increase the pressure drop when ash deposition was beyond the value. The larger particle size of ash would lead to the lower pressure drop because the porosity of ash with the large particle size was greater than that with small particle size and the amount of deposited soot through in the micropores was relatively little due to the film effect of large particle. The pressure drop of α-Al₂O₃ was relatively larger than that of cenospheres because the cenospheres was a mixture of SiO₂ and Al₂O₃. Pure Al₂O₃ was easy to deposit due to its small diameter. The small particle size of ash was similar to engine emitted soot, which deposited in the micropores of DPF and prevented the entering of smaller soot.