The invention relates to a wet ammonia flue gas purification process for the combined sulfur and mercury removing and the system thereof. The process adopts chlorine gas as oxidizing agent to completely oxidize elemental mercury in the flue gas, and gas phase divalent mercuric ion after being oxidized is converted into liquid phase divalent mercuric ion through the improved wet ammonia desulphurization process, and further the liquid phase divalent mercuric ion is converted into stable mercuric sulfide sendimentation through liquid phase divalent mercuric ion stabilizing agent. The system comprises a flue gas oxidizing demercuration preprocessing system composed of an oxidizing agent injection device and an electrostatic precipitator, a wet ammonia flue gas sulfur and mercury removing system composed of a reaction absorbing tower, a sulfur removing accessory substance processing system composed of a cyclone cleaner, a dewaterer, and a dryer, and a mercuric sulfide HgS sendimentation processing system composed of a wastewater buffer pool, a liquid phase mercuric ion reaction chamber, and a settling pond. The process and the system can effectively improve the oxidation rate of the elemental mercury in the flue gas, overcome the defaults that the divalent mercuric ion is easy to be reduced and released and the secondary pollution problem is caused by excess oxidizing agent, and greatly improve the efficiency of sulfur removing and mercury removing.