  We report the transit observations of the ultra hot Jupiter WASP-121b using
the Goodman High Throughput Spectrograph (GHTS) at the 4-meter ground-based
telescope Southern Astrophysical Research Telescope (SOAR), covering the
wavelength range $502-900$ nm. By dividing the target and reference star into
19 spectroscopic passbands and applying differential spectrophotometry, we
derive spectroscopic transit light curves and fit them using Gaussian process
framework to determine transit depths for every passbands. The obtained optical
transmission spectrum shows a steep increased slope toward the blue wavelength,
which seems to be too steep to be accounted for by the Rayleigh scattering
alone. We note that the transmission spectrum from this work and other works
differ obviously from each other, which was pointed out previously by
\citet{Wilson2021} as evidence for temporal atmospheric variation. We perform a
free chemistry retrieval analysis on the optical transmission spectra from this
work and the literature HST/WFC3 NIR spectrum. We determine TiO, VO and
H$_{2}$O with abundances of $-5.95_{-0.42}^{+0.47}$ dex,
$-6.72_{-1.79}^{+0.51}$ dex, and $-4.13_{-0.46}^{+0.63}$ dex, respectively. We
compare the abundances of all these three molecules derived from this work and
previous works, and find that they are not consistent with each other,
indicating the chemical compositions of the terminator region may change over
long timescales. Future multi-epoch and high-precision transit observations are
required to further confirm this phenomena. We note that when combining the
transmission spectra in the optical and in NIR in retrieval analysis, the
abundances of V and VO, the NIR-to-optical offset and the cloud deck pressure
may be coupled with each other.
