We propose a method for retrieving a beam wavefront from its near-field intensity distribution after a 4f system by simply inserting a benchmark at the Fourier plane. Through a convolution of the mark-blocked spatial frequency component and the original optical field with the 4f system, the separation between the focus of any sub-aperture and the benchmark can be determined to reconstruct the beam wavefront. Theoretical and experimental studies demonstrate the validity of this method, which has potential applications in real-time wavefront sensing.