In this paper, the Raman scattering spectra of cold atomic gas in different environments are closely calculated. Comparing the spectra from the atoms in free and trapped environments, the calculation result gives a right spectrum structure for the free atoms and reveals a distinct high-gain spectrum exhibiting a comb-like structure for the harmonic trapped atoms. The spectral peaks of the trapped spectrum are uniformly distributed and the distance between adjacent spectral peaks exactly equals the frequency of the trap. The nonlinear gain scheme and the gain conditions of the Raman scattering process are clearly given in this paper.