The three dimensional density distribution of a spin-polarized Fermi gas in a harmonic magnetic trap is calculated by using the Tomas-Fermi approximation. By fit the optical density distribution of absorption imaging,the total atom number,Fermi temperature and the actual temperature of the gas may be extracted. According this method,the optical density distribution of absorption imaging of degenerate fermions 40K in our experiment is fitted to obtain quantum degeneracy parameter T/TF of the Fermi gas.