Sound field separation technique has been proposed, which overcomes the limitati on on applications of near_field acoustic holography (NAH). The limitation is th at the sound field on one side of holographic plane must be free，that is to say , all the sound sources must be confined to the other side. Sound field separation technique with double holographic planes has been built according to the wave field extrapolation theorem and the superposition theorem of sound pressure scalar in wave_number domain, and the technique with single holographic planes has also been built according to Euler's formulation and the vector superposition theorem of particle velocity in wave_number domain. A particular feature of the technique is that the influence of background noise on reconstructed results can be removed while retaining the background noise in the holographic measurements. The derivation of the principle verifies the technique theoretically, and the computer simulations and experiment demonstrate its feasibility and effectiveness.