Here, we selected macromolecular polyallylamine (PAH) as the monomer in an aqueous-phase reaction for the first time, which underwent interfacial polymerization with 1,3-benzenedisulfonyl chloride (BDSC) on the surface of a polyethersulfone (PES) ultrafiltration membrane to prepare a new PSA composite membrane with positive charge, acid stability and high separation performance. By tailoring the polymerization conditions, the desired PSA composite membrane exhibited excellent rejection of different salts [MgCl₂ (92.44%) > MgSO₄ (89.2%) > NaCl (56.8%) > Na₂SO₄ (55.2%)] and a high permeation flux of up to 34.10 L m⁻² h⁻¹ at 0.5 MPa. The properties of the membrane were evaluated using various characterization techniques. The results indicated that the new PSA membrane is more positively charged and more compact than reported PSA composite membranes. In addition, it exhibited high acid stability. After exposure to a 20% (w/v) H₂SO₄ solution for 30 days, the MgCl₂ rejection level reached 88.3%. Finally, we used the new PSA composite membrane to test some heavy metal ions and found that the rejection level was always greater than 90%. Therefore, the new PSA composite membrane exhibited potential for water desalination and the removal of heavy metal ions from an acidic environment.