Knockout reaction experiment was carried out by using the He-6 beams at 61.2 MeV/u impinging on a CH2 target. The alpha core fragments at forward angles were detected in coincidence with the recoiled protons at larger angles. From this exclusive measurement the valence nucleon knockout mechanism and the core knockout mechanism can be distinguished by the relation between the polar angles of the core fragments and the recoiled protons, respectively. It is demonstrated that the core knockout mechanism may result in some strong contamination to the real invariant mass spectrum.