Nonthermal plasma can achieve diesel particulate filter regeneration under much lower temperatures (≤200 °C), showing promise for potential applications. To investigate the evolution of particulate matter, partial regeneration of a filter was produced by active substances generated by a plasma injection system. The surface morphology, oxidation characteristics, and surface functional groups of particle samples inside the filters before and after regeneration were investigated using digital electron microscopy, scanning electron microscopy, thermal gravimetric analysis, and fourier transform infrared spectrometry. Results show that during the regeneration step, the soot cake surface changed from random irregular cracks to more uniform and relatively dense pores. Decreases in characteristic temperatures and the growth of oxygen-containing functional groups indicated that nonthermal plasma could improve the oxidizing activity of particulate matter. The reaction between nonthermal plasma and particulate matter was most intense but not limited to the regeneration interface area of the soot cake.