The invention relates to a nitrogen-chlorine gas mixing system for purifying aluminum liquid and an aluminum liquid purification method. The system comprises a chlorine gas loop and a nitrogen gas loop, wherein the chlorine gas loop is a pipeline connected with a chlorine gas source and connected to a gas mixing joint sequentially via a valve I, a pressure sensor I, a chlorine gas pressure relief valve, a pressure gauge I, an on-off controller I and a flowmeter I for flow regulation; the nitrogen gas loop is a pipeline connected with a nitrogen gas source and connected to the gas mixing joint sequentially via a valve II, a pressure sensor II, a nitrogen gas pressure relief valve, a pressure gauge II, an on-off controller II and a flowmeter II for flow regulation; and the two gases are mixed by the gas mixing joint and enter a degassing apparatus for purifying aluminum liquid. The invention also relates to an aluminum liquid purification method. By using the scheme above, the invention implements and provides a secure and quantitative nitrogen-chlorine gas mixing system capable of monitoring leakage in the working process. The invention stabilizes the purification quality of the aluminum liquid and lowers the production cost.