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We present time-series echelle spectra of the Paβ line of the T Tauri star SU Aur, observed over three consecutive nights. The line shows strong variability (~10 per cent) over the velocity range 100-420 km s^-1 in the red broad absorption component, and weaker variability (~2 per cent) over the velocity range -200-0kms^-1 in the blue wing. The variability in the velocity range -200-0kms^-1 is correlated with that in 200-400kms^-1, and the variability in these velocity ranges anticorrelates with that in 0-100kms^-1. The mean spectrum from the second night shows the suggestion of a blueshifted absorption component at about -150kms^-1, similar to that found in the Hα and Hβ lines. We find the position of the subpeak in the red absorption component changes steadily with time, and its motion modulates at half the rotational period. We also find that the modulation of the line equivalent width is possibly associated with a half and a third of the rotational period, which is consistent with the surface Doppler images of SU Aur. Radiative transfer models of a rotationally modulated Paβ line, produced in the shock-heated magnetospheric accretion flow, are also presented. Models with a magnetic dipole offset reproduce the overall characteristics of the observed line variability, including the line equivalent width and the motion of the subpeak in the red absorption trough.