A principle, method and result of phase conjugation fidelity measurement based on “energy in the bucket”is psented. It is easy to obtain conjugation fidelity and realize the system calibration by measuring directly energy transmission ratio of spatial filter's pinhole with proper diameter. Choice criteria of pinhole diameter, coefficient calibration method, calculation formula of conjugation fidelity and experimemtal result of stimulated Brillouim scattering (SBS) phase conjugation mirror (PCM) with single cell under several kinds of situations have been provided. Finally, we point out its practical importance and defection．