The seismic diffraction wave is the seismic responses of subsurface small-scale discontinuous structures, such as fractures, faults, karst caves and pinching-out strata. Correct recognition of diffraction wave is of great significance for exploration of complex fault block oil and gas fields. Based on the characteristics of diffracted and reflected wave signals, a method for separation of diffracted wave is developed in this paper on the basis of plane wave prediction and the analysis of the influences of noise and plane wave prediction filter parameters. The plane wave prediction method can separate seismic reflections and diffraction waves by predicting the event of reflection wave with better continuity and retain the event of discontinuous diffraction wave with greater bending degree through dip estimation. The comparison of simulation data and actual data shows that noise seriously affects the separation of diffracted wave, and the separation result will be inaccurate if the noise is too large; the noise and false image will be introduced if the smooth radius of plane wave prediction filter is too small, and the separation of diffracted wave will not be complete if the smooth radius is too large, and the diffracted wave response of small fault block is difficult to be separated.