We experimentally investigate the gain spectra of a pair of conjugate beams by stimulated degenerate four wave mixing (DFWM) in cesium vapor. The results show that in a degenerate two-level transition system, the optical amplification effect based on stimulated DFWM can occur only when the angular momentum of ground state is greater than or equal to that of excited state. After considering all Zeeman sub-states of atoms, the qualitative theoretical analysis shows that the process is based on multiple N-type transition loops under the premise of stable ground state Zeeman coherence and large population of atoms at the initial Zeeman level. The effects of experimental parameters such as pump power, atomic number density and single photon detuning on the gain of DFWM signals are further analyzed. Moreover, it is found that the amplification efficiency of DFWM can be significantly improved by introducing an additional driving beam with a wavelength of 852 nm.