  We investigate emission line galaxies across cosmic time by combining the
modified L-Galaxies semi-analytical galaxy formation model with the JiuTian
cosmological simulation. We improve the tidal disruption model of satellite
galaxies in L-Galaxies to address the time dependence problem. We utilise the
public code CLOUDY to compute emission line ratios for a grid of HII region
models. The emission line models assume the same initial mass function as that
used to generate the spectral energy distribution of semi-analytical galaxies,
ensuring a coherent treatment for modelling the full galaxy spectrum. By
incorporating these emission line ratios with galaxy properties, we reproduce
observed luminosity functions for H$\alpha$, H$\beta$, [OII], and [OIII] in the
local Universe and at high redshifts. We also find good agreement between model
predictions and observations for auto-correlation and cross-correlation
functions of [OII]-selected galaxies, as well as their luminosity dependence.
The bias of emission line galaxies depends on both luminosity and redshift. At
lower redshifts, it remains constant with increasing luminosity up to around
$\sim 10^{42.5}\rm \, erg\,s^{-1}$ and then rises steeply for higher
luminosities. The transition luminosity increases with redshift and becomes
insignificant above $z$=1.5. Generally, galaxy bias shows an increasing trend
with redshift. However, for luminous galaxies, the bias is higher at low
redshifts, as the strong luminosity dependence observed at low redshifts
diminishes at higher redshifts. We provide a fitting formula for the bias of
emission line galaxies as a function of luminosity and redshift, which can be
utilised for large-scale structure studies with future galaxy surveys.
