Abstract: To address the challenge of the synergistic treatment of multiple pollutants, including ammonia, mercury, and alkaline aerosols, in pressure-relief off-gas generated during the high-temperature and high-pressure desorption and electrowinning of gold-loaded carbon, a four-stage, process-wide alkaline synergistic purification process was developed based on a theoretical investigation of the process. The process comprises "Stage Ⅰ condensation, Stage Ⅱ chemical absorption, Stage Ⅲ scrubbing, and Stage Ⅳ adsorption." Magnesium ammonium phosphate （MAP） precipitation and an alkaline peroxide-sulfide composite mercury-removal reagent were adopted from other fields and optimized to immobilize gaseous ammonia through chemical precipitation and achieve deep mercury removal. Industrial test results showed that the process achieved significant pollutant removal. The mass concentration of ammonia in the discharged off-gas decreased from 376 mg/m³ to 2.62 mg/m³, corresponding to a removal efficiency of 99.30 %. The mercury concentration remained consistently below 0.01 mg/m³, with a removal efficiency exceeding 99.5 %. In addition, the synergistic mechanism through which "deep ammonia immobilization enhances mercury precipitation" was revealed, and a pH range of 8.50−9.0 was identified as optimal for synergistic performance and safe operation, thus fundamentally preventing the formation of HCN and H₂S. The process features high treatment efficiency, stable operation, no secondary pollution, and economic viability, providing a reliable technical solution for the advanced treatment of similar off-gases in the industry.