A 2D velocity field of the eruptive prominence (EP) of 1991 March 5 is obtained from its spectral data observed at the Yunnan Observatory and the velocity distributions along the entrance slit are derived for different observing frames. Under the assumption that matter in the EP undergoes axial, radial and possible rotational motions, we construct a theoretical velocity distribution of the EP along the entrance slit, to derive, by fitting, the angular velocity of rotation ω and the other three parameters (axial velocity v0, radial velocity vr and the angle between the EP plane and the line of sight φ). We found: an averaged angular velocity ω of 3.0 × 10-3 arc s-1 and the variation of ω with the height above the solar limb. As the EP rises, the matter within it in fact moves along a spiral path around its axis. The spiral motion density gradient between the eruptive prominence and the surrounding corona. A theoretical angular velocity w is estimated based on the DPD and basically coincides with w obtained from the optimal velocity fitting.