By means of time-evolution operator,the emission spectrum of a Λ-type three-level atom interacting resonantly with two-mode thermal radiation cavity fields was studied.The physical spectrum expression of radiation emitted by the atom was given.The structure of the emission spectrum for a thermal input was discussed.The results indicate that whether two lower levels are degeneracy or not,Rabi splitting occurred in the emission spectra of Λ-type three-level atom when the average photon number of two-mode thermal radiation cavity fields is very small.And the structure of the resonant emission spectra of the atomic system has ten peaks generally.