  Damped Lyman-$\alpha$ Absorber(DLA) of HI 21cm system is an ideal probe to
directly measure cosmic acceleration in real-time cosmology via
Sandage-Loeb(SL) test. During short observations toward two DLAs in the
commissioning progress of FAST, we manage to exhibit an HI 21cm absorption
feature from PKS1413+135 spectrum in one epoch with our highest resolution up
to 100 Hz, preliminarily validating the frequency consistency under different
resolutions and bandwidths. We make a Gaussian fitting to extract the spectral
features, introduce two theoretical indicators to describe the fitted velocity
uncertainty, and ultimately give a mean redshift and its constraint of
$z_\mathrm{M}=0.24670045\pm0.00000036$ in accord with most literature. But our
redshift error of the target is still three magnitudes higher than the level we
can reach the drift signal. Though our first preparation has some flaws in time
recording and diode settings, it still proves the correctness of our data
process. Confined by limited observing time, we do not strech FAST's ability to
obtain a better velocity constraint, so further researchs are needed and in
schedule. With fine sensitivity and improving spectral resolution, such
observations in FAST could have reasonable possibility to explore cosmic
acceleration in late time universe practically.
