The experimental observation of the absorption spectrum of cold atoms in magneto-optical trap (MOT) was reported. Because the Doppler broadening of cold atoms in MOT is far less than the hyperfine structure splitting, three abosrption peaks (Cs,6S1/2,F=4→6P3/2,F=3,4,5) with a ratio of absorption coefficient of 3:7:12 were observed. The density of cold atoms was also measured by using absorption signal and the result agreed with fluorescence detecting method within 10% tolerance. The temperature of cold atoms measured using absorption method agreed well with that by fluorescence TOF method and release and recapture method.