Abstract: Based on the basic principle of SP log and the properties of flooding bed, from the viewpoint of electric circuit, the concept of current increasing factor and the basic principle of SC (spontaneous current) lag are presented. There are two types of flooding models, i. e., partial flooding and multist-age flooding models. The vertical variation of diffuse membrane potential of the flooding bed is given. Using formation superposition method, the theoretical formula of annular spontaneous potential and current of the flooding reservoir is derived. The theoretical curves for different flooding conditionts are plated with computer.