  The physical composition of the ejecta of gamma-ray bursts (GRBs) remains an
open question. The radiation mechanism of the prompt gamma rays is also in
debate. This problem can be solved for the bursts hosting distinct thermal
radiation. However, the events with dominant thermal spectral components are
still rare. In this work, we focus on GRB 220426A, a recent event detected by
Fermi-GBM. The time-resolved and time-integrated data analyses yield very hard
low-energy spectral indices and rather soft high-energy spectral indices. This
means that the spectra of GRB 220426A are narrowly distributed. And the
Bayesian inference results are in favor of the multicolor blackbody (mBB)
model. The physical properties of the relativistic outflow are calculated.
Assuming a redshift $z= 1.4$, the bulk Lorentz factors $\Gamma$ of the shells
are found to be between $274_{-18}^{+24}$ and $827_{-71}^{+100}$, and the
corresponding photosphere radii $R_{\rm ph}$ are in the range of
$1.83_{-0.50}^{+0.52} \times 10^{11}$ and $2.97_{-0.15}^{+0.14} \times 10^{12}$
cm. Similar to GRB 090902B, the time-resolved properties of GRB 220426A satisfy
the observed $\Gamma-L$ and $E_p-L$ correlations, where $L$ is the luminosity
of the prompt emission and $E_{p}$ is the spectral peak energy.
