In this paper, water usage of Beijing was forecasted by combining trend forecasting method and water quota method. The empirical frequency under different precipitation was given by the use of the Pearson Hydrology Frequency Curve. The changes of water resources shortage quantity and risk after “South-to-North Water Transfer” Project were calculated. The mainly conclusions are as follows. Water usage of Beijing is guaranteed sufficiently under the normal annual after “South-to-North Water Transfer” Project. In dry years, “South-to-North Water Transfer” Project reduces water shortage over the proportion of 60%, in the average year over about 40%, in wet years without water shortage. The effect of the project to reduce water shortage is more obvious in dry years and is growing with the time going. Risk of water shortages in Beijing in during 2015 -2030 after the implementation of the Project was reduced from 70% -80% to 26% -36% , totally above a 40% reduction. The project decreases more and more water shortage risk with time.