  XTE J1810-189 underwent an outburst in 2008, and was observed over $\sim 100$
d by RXTE. Performing a time-resolved spectral analysis on the photospheric
radius expansion burst detected on 2008 May 4, we obtain the source distance in
the range of 3.5--8.7 kpc for the first time. During its outburst, XTE
J1810-189 did not enter into the high/soft state, and both the soft and hard
colours decreased with decreasing flux. The fractional rms remained at high
values ($\sim 30$ per cent). The RXTE/PCA spectra for 3-25 keV can be described
by an absorbed power-law component with an additional Gaussian component, and
the derived photon index $\Gamma$ increased from $1.84\pm0.01$ to $2.25\pm0.04$
when the unabsorbed X-ray luminosity in 3-25 keV dropped from $4\times10^{36}$
ergs s$^{-1}$ to $6\times10^{35}$ ergs s$^{-1}$. The relatively high flux,
dense observations and broadband spectra allow us to provide strong evidence
that the softening behaviour detected in the outburst of XTE J1810-189
originates from the evolution of non-thermal component rather than the thermal
component (i.e. neutron star surface emission).
