  The Chinese CubeSat Mission, Gamma Ray Integrated Detectors (GRID), recently
detected its first gamma-ray burst, GRB 210121A, which was jointly observed by
the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor
(GECAM). This burst is confirmed by several other missions, including \fermi
and \textit{Insight}-HXMT. We combined multi-mission observational data and
performed a comprehensive analysis of the burst's temporal and spectral
properties. Our results show that the burst is relatively special in its high
peak energy, thermal-like low energy indices, and large fluence. By putting it
to the $E_{\rm p}$-$E_{\rm\gamma, iso}$ relation diagram with assumed distance,
we found this burst can be constrained at the redshift range of [0.3,3.0]. The
thermal spectral component is also confirmed by the direct fit of the physical
models to the observed spectra. Interestingly, the physical photosphere model
also constrained a redshift of $z\sim$ 0.3 for this burst, which help us to
identify a host galaxy candidate at such a distance within the location error
box. Assuming the host galaxy is real, we found the burst can be best explained
by the photosphere emission of a typical fireball with an initial radius of
$r_0\sim$ 3.2 $\times 10^7$ cm.
