  GRB 230307A is one of the brightest gamma-ray bursts detected so far. With
the excellent observation of GRB 230307A by Fermi-GBM, we can reveal the
details of prompt emission evolution. As found in high-time-resolution spectral
analysis, the early low-energy spectral indices ($\alpha$) of this burst exceed
the limit of synchrotron radiation ($\alpha=-2/3$), and gradually decreases
with the energy flux ($F$). A tight $E_{\rm p}\propto F^{0.54}$ correlation
anyhow holds within the whole duration of the burst, where $E_{\rm p}$ is the
spectral peak energy. Such evolution pattern of $\alpha$ and $E_{\rm p}$ with
intensity is called ``double tracking". For the $\alpha-F$ relation, we find a
log Bayes factor $\sim$ 210 in favor of a smoothly broken power-law function
over a linear function in log-linear space. We call this particular $\alpha-F$
relation as broken ``$\alpha$-intensity", and interpret it as the evolution of
the ratio of thermal and non-thermal components, which is also the evolution of
the photosphere. GRB 230307A with a duration of $\sim 35~\rm s$, if indeed at a
redshift of $z=0.065$, is likely a neutron star merger event (i.e., it is
intrinsically ``short"). Intriguingly, different from GRB 060614 and GRB
211211A, this long event is not composed of a hard spike followed by a soft
tail, suggesting that the properties of the prompt emission light curves are
not a good tracer of the astrophysical origins of the bursts. The other
possibility of $z=3.87$ would point toward very peculiar nature of both GRB
230307A and its late time thermal-like emission.
