Abstract: Before the MS 8.1 earthquake happened in November 14, 2001 in the western Kunlun Mountain Pass, the borehole strain gauge at Ku' erle station recorded a great strain change.After making sure the anomaly in the field several times and through argumentation, it has been affirmed that the anomaly is reliable.According to the data recorded at Ku′erle station, the relative stress field is calculated, analyzed and discussed.The result indicates that the principle stress direction before the MS 8. 1 earthquake in the western Kunlun Pass appeared change of deflection, the largest deflection amplitude extended 83 The change of principle stress reached 1400 kPa.The shearing stress rose by 523kPa.The basic characteristics of relative stress field are investigated and relevant questions are discussed.

Key words: Western Kunlun Pass 8.1 Earthquake, Bore-hole strain, Relative stress field, Deflection of stress direction, Measuring section in Ku′erle