Bifurcation of resonance trajectory of axial turbo compressor rotor blades is under study, which jumps to blade flutter. For this purpose a spectral analysis of blade transient oscillation records is performed. As shown, in contrast to resonance case of constant phase difference between excitation and induced blade oscillations, phase shift of blade collective synchronous oscillations under flutter depends on blade number and diametral mode of blade cascade. Blade flutter mode relates on a diametral mode of air flow pressure pulsation.