Degree-day is a quantitative index demonstrated to reflect the demand for energy to heat or cool houses and businesses. In this paper, the 18°C threshold temperature is chosen in Beijing. The homogenized mean daily temperature series during 1960-2009 in Beijing Weather Observatory is collected to calculate HDD and CDD. The mean monthly maximum HDD is 672.5 degree-days in January and zero in June, July and August. The annual mean value of HDD equals 2865.9 degree-days. There is a decreasing trend of - 99. 6 degree-days per decade. The mean monthly maximum CDD is 260.8 degree-days in July. All CDD in January, February, March, November and December are zero. The annual mean value of CDD in warm season is 849.3 degree-days. There is an increasing trend of 35.1 degree-days for CDD per decade. The annual HDD, CDD are highly correlated with the annual temperatures. With the temperature warming in Beijing, the smaller the HDD, the less the energy consumption in winter, the larger the CDD, the more the energy consumption in summer in the future.