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High-speed PIV is first used to measure the fluctuating particle velocities of a horizontal self-excited pneumatic conveying at low air velocity of minimum pressure drop in the acceleration and fully-developed regimes. Then, an orthogonal wavelet multi-resolution analysis and power spectrum are employed to reveal multi-scale characteristics of particle fluctuation velocity. It is found that the pronounced peaks of the spectra of axial and vertical fluctuation velocities appear in the low frequency range near the bottom of the pipe, and the peaks of the spectra become larger by using fins. In the range of low frequencies (3–25 Hz), the wavelet components of the fluctuating energy of axial particle velocity are the main contributions, accounting for 35% and 91% for non-fin and using fins, respectively, near the bottom of the pipe. However, in the range of relatively high frequencies (50–400 Hz), the wavelet components of using fins, accounting for approximately 57%, become smaller than that of non-fin, accounting for approximately 66%, in the fully-developed regime near the top of the pipe. The auto-correlation with fins displays large quasi-periodical waves in the low frequency regime as τ increases. [ABSTRACT FROM AUTHOR]