We propose an optical diagnosis for plasma electron density measurement with the Coherent Modulation Imaging(CMI) method which is a kind of phase retrieval technique and doesn’t need a reference beam. The phase of the diagnostic light can be reconstructed by CMI algorithm. The distribution of electron density can be obtained by dealing with the phase change using Abelian integral formula. Experimental results showed that the area where the plasma generated is ejected from the center. As the energy of laser driver beam increases, the phase difference region resulting from the generation of the plasma becomes larger.