Abstract: A grey system model, the dynamic model GM(1,1) g model, for recognition of seismogeochemical precursor is studied. Dissolved hydrogen gas dynamic change in Langanshi hot well, Beijing is used in the study here. The seismic precursor was found 15 days before the Fengzhen earthquake, Nei Monggol on August 13, 1981, M s=5.8 ). The concept of recognition index M(t)for the dynamic precursor is firstly suggested in this paper. The index is more effective than the static index in recongniyion of seismic precursor.