The GMI effect of composite amorphous wire consisting of Ag core and outer NiFe shell was investigated. GMI effect can be observed at low frequency. The expression of the impedance is deduced when the applied field is lower than the anisotropy field. The influence of parameters on GMI effect was theoretically analyzed. It is found that the GMI effect is strongly dependent on the thickness of ferromagnetic layer. And the GMI effect increase considerably when the conductivity of the inner core is much larger than that of the shell.