Electron acceleration by a propagating short ultra-intense laser pulse in a low-density plasma has been investigated. Electrons have the maximum energy when meeting the peak of the laser pulse. If a propagating laser pulse is abruptly stopped by a solid target, the highly energetic electrons will continue to move forward inertially and escape from the laser field. The envelope of the laser pulse is taken into account and there is an optimal position between the electron and the solid target. The electron maximum energy depends on the laser intensity and initial electron energy, and has nothing to do with the polarization of the pulse, but a linearly polarized laser pulse is more effective to accelerate electron than circularly polarized one under the same laser energy. The influence of the reflected light has been taken into account which makes our model more perfect and the results give good agreement with particle in cell simulations.