Abstract: The maximum. minimum and mean temperature in Beijing since 1940 are selected.The general trend of change in these temperatures and the interannual change of temperatures are studied in this paper.The consequences are shown as follows:the linear rate of increase in minimum temperature in January is equal to 4.08℃/100a;the maximum temperature in July decrease with a linear rate -0.245℃/100a.So the diurnal temperature range (DTR) become small.In addition, a fact which is important for reserching on global warming has been discovered:the temperature warming occurs mainly in day time in 1940s;and the temperature increases in the night in 1980s.