The prompt phase emission mechanism of Gamma-Ray Bursts (GRB) is still an open question. The GRB spectrum is often well-fitted by an empirical smooth broken power-law function termed as the Band model. However, with the detection of a large population of GRBs, it was realised the presence of a thermal component along with the non-thermal counterpart is evident in the prompt spectra. To understand the prompt emission from GRB, we perform a detailed study of the relativistically expanding fireball model with its temperature evolving as a function of its radius. The numerical code developed under this scenario is coupled with XSpec as a local model and used to fit the time-averaged spectrum of a recently detected GRB 221206B by Fermi-GBM. We find the spectra can be fitted with an expanding fireball model in combination with a power law. Our fit results suggest the emission is dominated by the thermal process with photospheric radius $r_{ph}$ of $13.64_{-0.07}^{0.08}$ log cm and photospheric temperature $T_{ph}$ of $658.5_{35.5}^{-50.4}$ keV.