To explore the changes of three-tone paradigm P300 and observe the improvement of the diagnostic sensitivity of P300 in aging dementia.Thirty-four patients of aging dementia underwent traditional P300 (2t-P300) and three-tone paradigm auditory P300 (3t-P300). The results of these two methods were compared.Abnormal P300 was found in 18 of 34 dementia cases (52.9%) by 2t-P300, and in 26 of 30 patients who finished the 3t-P300 tests (86.7%). In 2t-P300, 9 of 24 mild cases, 7 of 8 moderate cases and 2 severe cases showed abnormal results. In 3t-P300, 20 of 24 mild cases and all 6 moderate cases showed abnormal results. Abnormal 3t-P300 was found in 13 of 16 cases which showed normal 2t-P300. Besides prolonged and bad-shaped P300, no certain response of P300 to rare non-target stimulus (r-nT) was found in 10 mild cases by 3t-P300.Three-tone paradigm auditory P300 had higher sensitivity in early diagnosis of aging dementia than traditional auditory P300. No P300 response to r-nT in 3t-P300 is an important index in early diagnosis of mild dementia.