We present an analytical and numerical study of the fluorescence spectra of a bistable driven system by means of the Keldysh diagram technique in pseudoparticle representation. The spectra exhibit smooth transition between the ultraquantum and the quasiclassical limits and indicate the threshold value of the external field when changing of the most probable stable state occurs. The analysis of the fluorescence spectra also allows us to determine the most probable stable state of the system. It was also shown that, at integer and half-integer detuning-nonlinearity ratios, multiphoton resonance leads to abrupt changes in fluorescence spectra. It was also revealed that the fluorescence spectra are symmetric in the limit of zero environment temperature. The predicted features of the spectra could be observed in experiments with ultrahigh-quality resonators in either microwave or optical domains.