In order to explore the potential environmental risks associated with high-volume application of separated pig digested slurry and determine its optimal application rate, a two-year field experiment was conducted to investigate the effects of different application rates of pig slurry (complete substitution of chemical fertilizers) on the production performance Pennisetum purpureum and soil quality. The crude protein content, biomass, and heavy metal (Cd, As, Hg, Cu, Zn) accumulation characteristics of Pennisetum purpureum, as well as soil pH and nutrients (total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, and soil organic matter) were analyzed and compared. The results showed that: the concentrations of heavy metals Cd and Hg in the plant under all pig slurry application treatments were below the detection limit. Pig slurry application significantly increased the contents of As, Cu, and Zn in the plants, with the 200% chemical N equivalent treatment reaching peak values of 0.41, 13.49, and 38.31 mg/kg, respectively. However, all measured values were well below the limits specified in the Hygiene Standard for Feeds (GB 13078-2017) (As: 4.0 mg/kg;), indicating safe being used as forage grass. The effect of pig slurry on increasing soil potassium content was limited, but generally improved soil fertility. The most significant improving soil fertility was observed at the 200% N application rate, with notable increases in total nitrogen, alkali-hydrolyzable nitrogen, total phosphorus, available phosphorus, and organic matter compared to the conventional fertilizer control, with the increase rates being 45.24%, 42.94%, 42.20%, 74.77% and 55.91% respectively. The experimental area exhibited a naturally high background level of Cd in the soil, and Cd accumulation was observed following pig slurry application. However, the contents of the other four heavy metals all met the requirements of the risk screening values for soil contamination of agricultural land specified in GB 15618-2018. Under the experimental conditions, high-volume pig slurry application (≤200% N) in Pennisetum purpureum cultivation systems is proved feasible, maintaining yield and forage quality while enhancing soil fertility and keeping environmental risks manageable, thus, it is recommended to be used in the study area to achieve a balance between production efficiency and ecological safety.