In this study, to reduce the nitrogen loss during pig slurry storage and improve the safety of returning pig slurry to the field, acidification storage technology was used and H₃PO₄ was used as acidifier to study the effects of different initial pH on the acidification storage process and nitrogen (N) loss of pig slurry. The results show that the order of contents of heavy metals in pig slurry are Cu > Pb > Zn > Cd > As, and the contents of heavy metals are decreased after 180 days of storage which are below the limited values of GB/T 40750—2021 ‘Agricultural Biogas Liquid'. However, nitrogen in pig slurry is lost after 180 days of storage, and N loss is up to 68.55%. The main nitrogen form in stored pig slurry is ammonia nitrogen (NH₃-N), accounting for 51.73% of total nitrogen. The correlation formula between acidification pH and acidifier dosage is y = –3.3113x + 22.999, R² = 0.985. Compared to CK, N loss of acidified pig slurry is greatly reduced by 5.98–62.77 percentage points, and nitrogen retention effect is inversely proportional to acidification pH. Meanwhile, the proportion of NH₃-N to TN in acidified pig slurry is increased up to 24 percentage points. The concentrations of total phosphorus and soluble phosphorus in pig slurry are increased by acidification storage with H₃PO₄ and the increase is positively correlated to the addition amount of H₃PO₄. After acidification storage, EC, contents of Cd and Pb in pig slurry are high, which inhibit the growth of roots and stems of wheat, and its negative effect is inversely proportional to acidification pH, while Cu content is reduced and is positively correlated to acidification pH. There is no obvious regularity of different acidification pH on As and Zn contents of pig slurry. In conclusion, adjusting pig slurry to pH 6.0 is more appropriate for acidification storage with the acidifier cost of 13.89 yuan per ton.