Abstract: On the basis of analyzing UV observation data,it is put forward that substances in the atmosphere can absorb UV energy.The calculated UV energy loss in the actual sky compared to clear sky in 1990 was 64～109 W/m\+2,this UV energy loss attenuated by ozone,“water vapor" and aerosol was 39 1～66 3,21 5～36 5 and 3 2～5 5 W/m\+2 respectively (except December), and yearly averaged values were 48 3, 26 6 and 4 0 W/m\+2 respectively.The result shows that cloudy atmosphere absorbs UV energy,and there is 26 6 W/m\+2 lost in the cloudy atmosphere,because of the role of “water vapor",i.e.the UV energy absorption by substances in the atmosphere when they take part in photochemical reactions.The calculated value agrees with the observed and modeled results abroad.Because existing models have not taken this part of energy into full consideration,there is a significant difference between modeled and observed results.