A dual cosinoidal phase modulation method is proposed to improve spectrum compression of the super-Gaussian pulse with nonlinear chirp in a highly nonlinear fiber. The results show that the temporal unlevel phase of the super-Gaussian pulse resulting from the mismatch between the initial nonlinear negative chirp and the nonlinear positive chirp that induced by self-phase modulation can give rise to the spectrum splitting and residual sidelobes, which degenerates the quality of spectral compression. Phase compensation by external dual cosinoidal phase modulation can effectively reduce the residual sidelobes in the spectrum and extend the distance of spectral compression, so that the quality of the spectrum compression can be enhanced. And the amplitudes and frequencies of the dual cosinoidal phase modulation originate from the nonlinear phaseshift analysis for the pulse. Also, the quality of spectrum compression in a wide range of the initial stretching factor can be optimized.