Abstract: Ultraviolet Index (UVI) was measured at Beijing Observatory between May 1999 and October 2001.The results show that the UVI varies significantly in different seasons.The maximum of UVI is 7\^85,9.87,4.92 and 2.99 in spring,summer,autumn and winter,and the seasonal averaged UVI at noon is 3.52,5.31,2.17 and 1.24 respectively.The UVI reaches its peak between 10:00 and 15:00 in every day.And during the period of time,the UVI is generally in the range of 0～2 in autumn and winter,which has less harm to human health.But in spring,the mean accumulative irradiation time is 159,87,47 and 2 minutes,and 75,99,93 and 29 minutes in summer for the UVI of 0～2,3～4,5～6 and above 7,respectively.So the ultraviolet radiation in spring and summer is very detrimental to human health.In addition,the accumulative distribution of UVI shows that the majority of UVI is below its mean value in spring,autumn and winter,but in summer,around noon most UVI is above its mean value.