Abstract: The new method of quality judgment about on-line aluminum alloy spot welding was studied, which was based on the rough set theory. The eight characters selected from two parameters (electrode displacement and electrode force in the weld process) constituted the knowledge representation system. The new method dealt with the obtained information by adopting the discretized continuous attribute algorithm with the self-organizing feature map network(SOM) and the attribute reduction algorithm based on discernibility matrix, picked up the distinguish rule, then completed the classification of spot welding quality by the rule. The method can not only reduce the dimensions of the feature space, the workload of quality classification and the information memory capacity, but also can make the accuracy of the spot weld quality judgment reach 97.03%.