Two-dimensional PIC (Particle-in-Cell) simulation is used to investigate the electron acceleration process when a vacuum channel is filled with pre-plasma. Using a tightly focused ultra-intense short-pulse laser to interact with a hollow plasma channel is an effective way to obtain a relativistic electron beam with high power and high collimation. In the experiment, the pre-plasma generated by the laser pre-pulse ablation of the target wall will expand and fill the vacuum channel, resulting in changes in the quality of the electron beam. The simulation results show that under the condition of short-pulse laser with a power density of 5.0 $ \times {10}^{20}\;{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2} $, the laser field preferentially interacts with the low-density plasma when the channel is filled with plasmas. The laser pulse interacts with the low-density plasma and the interaction of the channel wall is weakened. The electron acceleration mechanism is transformed from the vacuum electron acceleration dominated by the longitudinal field to the plasma electron acceleration dominated by the transverse electric field, resulting in an electron beam with a larger amount of charge, but with a lower energy and an increased divergence angle.