Abstract: Nowadays, oil and gas exploration is rapidly moving towards the areas with irregular topography and complex geologic structures. The technique of beamlet is often used for image making under such a circumstance. Beamlets have strong local and directional characters and can easily realize the migration of the whole structure and local illumination. "Wave field extension downward" is the method, which may easily avoid the influence of the irregular surface, to resolve the migration of the complex surface. Using the beamlet for prestack migration and illumination, the method is a combination of the beam source and Fourier propagator. At first, beam source synthesis is obtained from beam function in beamlet transform, and then the Fourier propagator is performed with wavefield extrapolation. The method is tested using an irregular surface model. The comparison of the migration results of a single directional beamlet and the beamlets with different window and directions demonstrates the validity of the method we proposed.