Atomic force microscope (AFM) based anodic oxidation is an important method to fabricate nano-structures and nano-devices. To realize precise fabrication, electric field between AFM tip and substrate should be under precise control. For precise control of the electric field, a necessary topic is to find out the distribution of the spatial electric field and the relationship between the electric field and parameters. By theoretical analysis we simulated the spatial distribution of the tip/substrate electric field and analyzed the relationship between the electric field and parameters, which were verified by experiments. Our work can provide theoretic support for electric field assisted nanofabrication.