We investigate the dynamics of coherent phonons in bismuth under high-density excitation with a pump-probe technique. As the pump power density is increased, the E g mode appears in addition to the A 1 g mode, and the A 1 g mode in the Fourier-transformed spectra is asymmetrically broadened. Analysis based on a time-partitioning Fourier transformation reveals that the frequency of the coherent A 1 g phonon is instantaneously downshifted upon the excitation and that the line shape of the A 1 g mode changes from an asymmetric to symmetric one as the coherent oscillation decays. These behaviors of the coherent phonons are attributed to anharmonicity of the lattice vibrations.