Efflorescence often appears on the city wall of Xian, forming large areas of white crystalline salts. Efflorescence causes cosmetic damage and can reduce the mechanical strength of the bricks, which leads to peeling and powdering, and even threatens the stability and safety of the wall. To investigate the cause of efflorescence, the crystalline salt was analyzed using XRD.The results showed that the major components are NaCl, NaNO₃, Na₂SO₄·10H₂O, and Na₂SO₄. Based on groundwater and rainfall data for the Xian area, the major cause of efflorescence is the introduction of soluble salts from construction materials, groundwater and rainfall. A conservation method against efflorescence was developed, first removing the surface salts with multiple layers of paper, following by sterilization and insulation.