  We analyzed RXTE/PCA observations of the bursting pulsar GRO J1744-28 during
its two outbursts in 1995-1997. We have found a significant transition, i.e., a
sharp change, of both spectral and timing properties around a flux of $\rm
10^{-8}\ ergs\ cm^{-2}\ s^{-1}$ (3-30 keV), which corresponds to a luminosity
of 2--8$\rm \times 10^{37}\ ergs\ s^{-1}$ for a distance of 4--8 kpc. We define
the faint (bright) state when the flux is smaller (larger) than this threshold.
In the faint state, the spectral hardness increases significantly with the
increasing flux, while remaining almost constant in the bright state. In
addition, we find that the pulsed fraction is positively related to both the
flux and the energy (<30keV) in both states. Thanks to the very stable pulse
profile shape in all energy bands, a hard X-ray lag could be measured. This lag
is only significant in the faint state (reaching $\sim$ 20\,ms between 3-4 keV
and 16-20 keV), and much smaller in the bright state ($\lesssim$2ms). We
speculate that the discovered transition might be caused by changes of the
accretion structure on the surface of the neutron star, and the hard X-ray lags
are due to a geometry effect.
