| In this paper, the occurrence and the characteristics of spatial and temporal distribution of atmospheric duct were known by analyzing the NCEP reanalysis data and conventional meteorological data as well as the weather systems and the atmospheric conditions in the South China Sea in 2006 ,moreover ,two daily representative time were selected in both spring and winter in order to carry out a detailed analysis of the pressure field, humidity field, temperature and humidity vertical profiles and the wind field and worked for studying the weather and environmental conditions at the time when the atmospheric duct Occurred. The characteristics of atmospheric boundary layer and the environmental change within the atmospheric boundary layer in the South China Sea were simulated with the mesoscale numerical model ARPS. And then by analyzing the vertical gradient of the amended atmospheric refractive index within the atmospheric boundary layer, the characteristics contained the emergence of atmospheric duct ,the layer depth, the type and the intensity were revealed.The results showed that: [1] In 110～140°E, and 5～35°N range of the South China Sea the atmospheric duct occurred the most frequent in April, not only the biggest number of days, but also a broad distribution, appeared in June at least, there are a large-scale atmospheric duct in December and January. Atmospheric duct in January was seen in the 12 o'clock, in April was seen in the 00 and the 06 o'clock, at the time of 12 o'clock more than when they appeared on the phenomenon of atmospheric duct, including one last month on the large-scale atmospheric duct In Jun , Oct and Dec ,the atmospheric duct occurred irregularly, mainly concentrated in the 06 pm and 12 pm, which does not appear in the 18:00. [2] In the spring, atmosphric duct mainly occured in the 130～140°E, 22～30°N, the region is located in the north-western of the anti-cyclones where affected by the influence of subsiding air, atmospheric duct is often accompanied with water vapor content with a high degree of sharp decline. Duct where the atmosphere is relatively stable atmospheric regions, there is often accompanied by low-level temperature inversion. In the winter ,the atmospheric duct occurred mainly in the 117～120°E, 15～19°N region. Atmospheric duct often seen at high pressure ridge duct line position, the small value of relative humidity Center, located in the northern part of the anti-cyclone, atmospheric wave guide within the regions in which the atmosphere is relatively stable. Atmospheric duct are located in a high incidence of anti-cyclonic atmospheric boundary layer near the top of the region. (3) Using the ARPS had a very good simulation of the atmospheric duct of the South China Sea. Simulation shows that the duct often seen in the eastern part of the South China Sea Islands and the region northeast of the western Pacific sea, as well as the eastern and north-east of the Philippines sea and the atmospheric duct in the north-west and the north-eastern edge of the anti-cyclone, it's location moving with the a closed anti-cyclone center ,and the intensity also be weaked when the center of the anti-cyclone disappeared. (4) Atmospheric duct layer is often accompanied by increase in temperature with height or the rapid increase in atmospheric water vapor specific humidity increases with height and decreases rapidly. In April, the bottom height of atmospheric duct layer from 80m to 200m, the thickness can reach about 300m. In June, the phenomenon of atmospheric duct less than other months, mainly located in the western Pacific sea surface east of the Philippines, and closed anticyclone center is located in the north-west. June duct layers of the atmosphere, in the case of June at the height of the bottom of the duct layer from 520m to 650m, the thickness can reach about 180m. In October, The top of duct layer at the height from 500 m to 700 m, the height of duct at the higher latitudes is lower than that at the lower latitude , the thickness of only about 100m. In January, the duct High-incidence area is located in the waters east of the South China Sea Islands, in the case show that the height of the bottom of duct layers from 380m to 520m, the thickness can reach about 140m.
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