The utility model relates to a variable-polarity plasma-arc welding nozzle. The linear profiles and acute angle connection manners of the traditional nozzles are difficult to further improve the plasma-arc deflection and difficult to realize the variable-polarity plasma-arc welding of the thick plate materials. The variable-polarity plasma-arc welding nozzle comprises an air inlet section L1, a compression section L2, a compression section L3 and an ejection section L4, wherein the air inlet section L1 and the compression section L3 are of linear structures; a thread (1) is arranged at the end of the compression section L3; a waterproof ring recess (2) is arranged onthe left side of the thread; the compression section L2 and the air inlet section 1 are connected in an arc transition manner; the compression section L3 and the compression section L2 are connected in an arc transition manner; the ejection section L4 adopts a conical structure. The utility model is applied to variable-polarity plasma-arc welding nozzles.