The image quality of the imaging system is often reduced by the wave-front aberrations which arises from a variety of sources including atmospheric turbulence, system aberration, and so on. Phase diversity image restoration is an image restoration technique that mainly aims to solve the problem of wave-front aberrations. This technique needs neither reference object nor the degraded function and has a good convergence. It can enhance the resolution and sharpness, increase the information content and improve the visual effect of the image, and has broad application prospect in the field of image restoration. In this paper, the phase diversity image restoration technique is studied, and its performance is analyzed in virtue of the concept of the optical transfer function, modulation transfer function and point spread function. Simulation and experimental results both show that phase diversity image restoration technique can not only reduce the influence of wavefront aberrations but also increase the information of low frequency within the cutoff frequency, and improve the contrast. So, the image acquired by phase diversity image restoration can be better than the image acquired by the diffraction limited system.