This paper aims to assist the individual clinical diagnosis of children with attention-deficit/hyperactivity disorder using electroencephalogram signal detection method. Firstly, in our experiments, we obtained and studied the electroencephalogram signals from fourteen attention-deficit/hyperactivity disorder children and sixteen typically developing children during the classic interference control task of Simon-spatial Stroop, and we completed electroencephalogram data preprocessing including filtering, segmentation, removal of artifacts and so on. Secondly, we selected the subset electroencephalogram electrodes using principal component analysis (PCA) method, and we collected the common channels of the optimal electrodes which occurrence rates were more than 90% in each kind of stimulation. We then extracted the latency (200~450 ms) mean amplitude features of the common electrodes. Finally, we used the k-nearest neighbor (KNN) classifier based on Euclidean distance and the support vector machine (SVM) classifier based on radial basis kernel function to classify. From the experiment, at the same kind of interference control task, the attention-deficit/hyperactivity disorder children showed lower correct response rates and longer reaction time. The N2 emerged in prefrontal cortex while P2 presented in the inferior parietal area when all kinds of stimuli demonstrated. Meanwhile, the children with attention-deficit/hyperactivity disorder exhibited markedly reduced N2 and P2 amplitude compared to typically developing children. KNN resulted in better classification accuracy than SVM classifier, and the best classification rate was 89.29% in StI task. The results showed that the electroencephalogram signals were different in the brain regions of prefrontal cortex and inferior parietal cortex between attention-deficit/hyperactivity disorder and typically developing children during the interference control task, which provided a scientific basis for the clinical diagnosis of attention-deficit/hyperactivity disorder individuals.