Abstract: By employing hydrodynamic theory, the interaction between the electron beam and the highfrequency field in the radial gap of the radial transittime oscillator is studied. By using numerical method, curves are given to illustrate the relation between the beamfield power exchange and the gap's transit angle. It shows that the cruuent density and the energy exchange will be oscillating with the transit angle increasing. The number density of beam is bigger, the energy exchange between the beam and the field is stronger. The results obtained from hydrodynamic theory are of more generality compared with those from single electron theory.