The jasper from Kunlun Mountain in Qinghai is analyzed with the method of situ high temperature X-ray diffraction(HT-XRD) analysis, which gives the result that the chemical formula of its main composition is Ca 2 Mg 5 [Si 4 O 11 ] 2 (OH) 2 at room temperature, proving to be Tremolite, still mixed with a spot of Dolomite and Talc. By analyzing the diffraction data, it is found that at 1150°C for the jasper's main composition phase transition occurs. This phase transition is that the Ca-Mg pyroxene forms because of the Tremolite being affected by products of the thermal decomposition of Dolomite and Talc.