  The radiation mechanism (thermal photosphere or magnetic synchrotron) and the
progenitor of gamma-ray burst (GRB) are under hot debate. Recently discovered,
the prompt long-duration ($\sim$ 10 s, normally from the collapse of massive
stars) property of GRB 211211A strongly conflicts with its association with a
kilonova (normally from the merger of two compact objects, NS-NS, NS-BH, or
NS-WD, duration $\lesssim$ 2 s). In this paper, we find the probability
photosphere model with a structured jet can satisfactorily explain this
peculiar long duration, through the duration stretching effect ($\sim$ 3 times)
on the intrinsic longer ($\sim$ 3 s) duration of NS-BH (neutron star and black
hole) merger, the observed empirical 2SBPL spectrum (with soft low-energy index
$\alpha$ of $\sim$ -1) and its evolution. Also, much evidence of the NS-BH
merger origin is found, especially the well fit of the afterglow-subtracted
optical-NIR light curves by the significant thermal cocoon emission and the
sole thermal red kilonova component. Finally, a convincing new explanation for
the X-ray afterglow plateau is revealed.
