Abstract: Mechanism for generation of geoelecfric precursors and a model of imperding earthquake precursorsof geoelectricity triggered by tidal forces are presented.The anomalies in both georesistivety and selfpotential measurements obtained two months before the M7.8 Tangshan earthquake of 1976 are taken as example, which have periods equal to that of the tidal forces M2 and MSf respectively, so that the reasonableness of the proposed model is proved and a new method of short-term or impending-earthquake prediction is proposed.According to the instability criteria of the stiffness (λ)comparison, the state of λ=0 is considered as the impending earthquake stage.In our model we regard the tidal force as the input signal while any field, which can directly or indirectly reflect the displacement of response is inversly proportional to the stiffness.Nomally the amplitude(A/λ) of response is always a very limited small value, which cannot be detected by our electric measurements, since λ is usually a relatively large positive constant during elestric deformation.However, the same small tidal force results in an abnormally large amplitude of theresponse, which is appoaching infinity when λ=0 and can be detectable, when an impending-earthquake stage is reached owing to sufficient accumulation of tectonic stress.The impending-earthquake response can be identified from noises according to identical and abnormally large amplitude to the tidal force and a short-term earthquake prediction can be made.