Abstract: By using the 40-year NCEP (1958～1997) grid point reanalysis meteorological data, we analyzed the inter-decade variation on the climatic characteristics of the onset of South China Sea summer monsoon. The results are: (1) There were great difference on the onset date of the SCS summer monsoon between the first two decades and the last two decades. It was late on 6^th pentad of May for the first two decades and was on 4^th and 5^th pentad of May for the next two decades. (2) Except for the third decade (1978～1987), the establishment of the monsoon rainfall was one to two pentads earlier than the onset of the summer monsoon in all other three decades. (3) The onset of the SCS monsoon is the result of the abrupt development and eastward advancement of the southwesterly monsoon over the Bay of Bengal. The analysis of four decades shows that there were abrupt development of the southwesterly monsoon over the Bay of Bengal between the 3^rd and 4^th pentad of May, but there was great difference between its eastward movement and its onset intensity. These may have important effect on the earlier or later onset of the SCS summer monsoon. (4) During the onset of the SCS summer monsoon, there were great difference of the upper and lower circulation feature between the first two and the nest two decades. At the lower troposphere of the first two decades, the Indian-Burman trough was stronger and the center of the subtropical high was located more eastward. At the upper troposphere, the northward movement of the center of subtropical high was large and located more northward after it landed on the Indo-China Peninsula. After comparing, we can see that the circulation feature of the last two decades was favorable to the establishment and development of the SCS summer monsoon.