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

Significant cocoon emission and photosphere duration stretching in GRB 211211A: a burst from a neutron star - black hole merger