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With the help of three-dimensional particle-in-cell simulations, we study the electron dynamics in radiation pressure acceleration of a mass-limited foil target.When a mass-limited foil is irradiated by a circularly polarized laser pulse, the wing parts of the laser overtake the foil from the foil boundaries.In the laser propagation direction, electrons are significantly accelerated by the laser radiation pressure.In the laser polarization direction, electrons are subjected to a strong radial space-charge force and oscillate in the overtaking laser field.Under their combined actions, electrons execute a betatron-like oscillation and a spatially helix electron bunch is thus formed.A simple analytical model is proposed for interpreting the high electron energies and large oscillating amplitudes.It is shown that, when the electron oscillation frequency coincides with the laser frequency as witnessed by the electron, betatron-like resonance occurs, resulting in hundreds of MeV electron generation.