In this paper, the problems about direct identification of precursor with the tilt vector are discussed, and the method is also put forward. On the basis of tilt modulus, the max-modulus of vector is researched. The stability of tilt vector and relationship between vector characters, (for exam pie:pause, knot. radical change) are studied. The relationships between the stability of vector characters and precursors of Gonghe. Qinghai and Jingtai, Gansu and Datong, Shanxi earthquakes are discussed. The results show that the knots and pauses may be steady, therefore they are uncertainly precursor marks; however, the radical change of vector is possible precursor. The research for stability is more meaningful and effective, for precursor analysis than direct method.