Ultrafine particulate matter (PM) is a critical part of PM2.5 and is of increasing concern due to its toxicity and controlling difficulty. In the coal-fired power stations, devices such as electrostatic precipitators are generally applied to reduce the PM emission; however, they cannot effectively capture the ultrafine PM. So, an in-furnace PM reduction technology has been proposed, which is supposed to reduce the formation of ultrafine PM during the coal combustion process by adding additives into the furnace. In this study, clay mineral attapulgite (ATT) was selected as the additive and a modified ATT was prepared via hydrochloric acid treatment. The natural and modified ATTs were separately added into coal powder and burned in a drop tube furnace at 1500 degrees C. The generated PM was sampled via a low-pressure impactor, and the mass yields, particle size distributions, compositions, and heavy metal (i.e., lead and vanadium) contents of the PM were determined via a microbalance, X-ray fluorescence probe, and microwave digestion followed by inductively coupled plasma mass spectrometry. The results showed that the natural ATT reduced the mass yield of ultrafine PM by similar to 28.8%. By contrast, the HCl-modified ATT reduced the ultrafine PM by similar to 44.1%, indicating a higher ultrafine PM reduction efficiency than the natural one. Moreover, adding ATT reduced the partitioning of heavy metal (e.g., lead, etc.) into the ultrafine PM and this performance was also improved (from 33.6 to 61.5%) via HCl modification. Further results suggested that the acid treatment removed the mineral impurities, exchangeable cations (e.g., Mg2+, Na+), and some Al in the attapulgite crystal structure, which improved the porosity, generated new adsorption sites, and promoted the formation of free silicate dioxide, and finally enhanced the capture performance of both ultrafine PM and heavy metals.