Illumination uniformity of a target is highly required to achieve central ignition in inertial confinement fusion based on direct-driven fusion. However, the shell may be compressed due to its interaction with laser beams during the laser pulse duration, and may further result in the degradation of illumination uniformity and the enhancement of the cross-beam energy transfer (CBET). Hence a novel fast zooming scheme for direct-driven laser fusion is proposed, i.e., by introducing the variable wavefront to the laser beam provided by a special-designed electro-optic crystal, thus the location and size of the focal spot could be controlled in real time, achieving the aim of improving the illumination uniformity and alleviating the CBET. This model of fast zooming has been built up, and variations of spot size and the radius of curvature of the additional spherical wavefront with time have been numerically simulated and analyzed based on the analysis of the interaction of the laser beam and the target. Results indicate that the fast zooming scheme could achieve effective and real-time control on the radius ratio between the focal spot and the target. Moreover, the fast zooming scheme has a little impact on the spatial filter capability and the third harmonic frequency conversion efficiency.