Spectrum of partially coherent polychromatic beams in laser resonators is studied. Analytical formulae for spectrum of Gaussian Schell-model beams in resonators are derived. Numerical results show that spectral shift generated by unstable resonators is much flat and smaller than that by stable resonators. Spectral switch takes place when mirror spot size increases to a certain order of magnitude. Results also show that dependence of generated spectrum on coherence length of sources is very low. These results may be useful for manufacturing high power laser resonators.