  In this paper, we investigate the star formation and chemical evolution of
damped Lyman-$\alpha$ systems (DLAs) based on the disc galaxy formation model
which is developed by Mo, Mao & White. We propose that the DLAs are the central
galaxies of less massive dark haloes present at redshifts $z\sim 3$, and they
should inhabit haloes of moderately low circular velocity. The empirical
Schmidt law of star formation rates, and closed box model of chemical evolution
that an approximation known as instantaneous recycling is assumed, are adopted.
In our models, when the predicted distribution of metallicity for DLAs is
calculated, two cases are considered. One is that, using the closed box model,
empirical Schmidt law and star formation time, the distribution of metallicity
can be directly calculated. The other is that, when the simple gravitational
instability of a thin isothermal gas disc as first discussed by Toomre is
considered, the star formation occurs only in the region where the surface
density of gas satisfies the critical value, not everywhere of a gas disc. We
assume that star formation in each DLA lasts for a period of 1 Gyr from
redshifts $z=3$. There is only one output parameter in our models, i.e. the
stellar yield, which relates to the time of star formation history and is
obtained by normalizing the predicted distribution of metallicity to the mean
value of 1/13 $Z_{\odot}$ as presented by Pettini et al. The predicted
metallicity distribution is consistent with the current (rather limited)
observational data. A random distribution of galactic discs is taken into
account.
