This paper, based on acoustic radiation, is focued on volume array. By using the finite element simulation software, the actual volume array model and the equivalent point source model are both established. Contrast and analysis of effect for mutual radiation to directivity of volume array. Mutual radiation impedance of the two models is calculated, which is varying with distance and frequency. Effect of mutual radiation is analyzed on resonance frequency, array work efficiency, bandwidth and so on. The results show that the point source model can predict the array property well and shorten the design cycle. Volume array of $\lambda /4$ gets severe mutual radiation influence. The radiation capability of the array is affected by the spacing of the array elements, the size of the array elements, the array configuration and the structure of the radiation surface.