  The star formation history (SFH) of galaxies is critical for understanding
galaxy evolution. Hydrodynamical simulations enable us to precisely reconstruct
the SFH of galaxies and establish a link to the underlying physical processes.
In this work, we present a model to describe individual galaxies' SFHs from
three simulations: TheThreeHundred, Illustris-1 and TNG100-1. This model
divides the galaxy SFH into two distinct components: the "main sequence" and
the "variation". The "main sequence" part is generated by tracing the history
of the $SFR-M_*$ main sequence of galaxies across time. The "variation" part
consists of the scatter around the main sequence, which is reproduced by
fractional Brownian motions. We find that: 1) The evolution of the main
sequence varies between simulations; 2) fractional Brownian motions can
reproduce many features of SFHs, however, discrepancies still exist; 3) The
variations and mass-loss rate are crucial for reconstructing the SFHs of the
simulations. This model provides a fair description of the SFHs in simulations.
On the other hand, by correlating the fractional Brownian motion model to
simulation data, we provide a 'standard' against which to compare simulations.
