Since the 17th century, large-scale indigenous zinc smelting activities have been carried out in northwestern Guizhou, China. Until the smelting activities were ceased in 2004, large amounts of zinc smelting waste slag were randomly stacked along the river and in the surrounding soil without taking any safe disposal measures have caused serious water, air, and soil contamination. Extremely harsh habitat conditions in waste slag such as poor physical structure, nutrient deficiency, and high toxic metals content, resulting devoid of plants in the waste slag site. Severe wind and water erosion in this area are the main ways for the diffusion of heavy metal in bare waste slag sites without vegetation. Phytostabilization, the most promising option for waste slag ponds reclamation, reduces heavy metal bioavailability, improves waste slag fertility, reduces wind and water erosion, and increase the diversity of vegetation and microbial community in waste slag. Particulate organic matter (POM, >53 μm) has attracted widely attention because it is a special type of organic matter that has a fast decomposition rate and is easy to enrich heavy metals. A field investigation showed that considerable organic matter (the thickness of the plant litter reached 3 cm) accumulated on the surface of the Zn smelting waste slag after 5 years of in situ aided phytostabilization. It is hypothesized that the POM formation after the litter decomposition could significantly impact the distribution of heavy metals in waste slag. Therefore, the objective of the present study was to investigate the interaction between POM and heavy metals in waste slag under in situ aided phytostabilization. The results showed that the input of litter has decomposed to different particle size fractions of POM, and the percentage of POM with a particle size of 0.05-0.25 mm was highest, followed by >1 mm and 0.5-1 mm, and the 0.25-0.5 mm was lowest. The mass of POM derived from four plants in the following order: Cryptomeria fortunei > Broussonetia papyrifera > Arundo donax > Robinia pseudoacacia. The content, enrichment coefficient, and mass load of heavy metals in the POM increase with the decrease of POM particle size, and the fine particle size fractions of POM with a particle size of 0.05-0.25 mm was the highest. The mass load of Cu, Pb, Zn, and Cd in POM in the following order: Cu>Cd>Zn>Pb. The surface of the coarse particle size POM is smoother, and the fine particle size POM is rougher and has a larger specific surface area. The main functional groups in different particle size fractions of POM were -COOH, -OH, C=O, C=C, and C-OH. The size and composition of POM fractions play a key role in determining the distribution of heavy metals in the waste slag. The finding in this study that the role of POM in heavy metal biogeochemical processes which have important implications for phytostabilization of waste slag.