Abstract: The liquid and plastic limits are the important factors to reflect the penetration resistance, anti-erosion and anti-shear of soils. The liquid ad plastic limits of the red clay in Guilin soaked in the acid and alkali solutions are tested to study its influencing factors and mechanism of boundary water content. The test results show that the plastic limit decreases and the liquid limit or plastic index decreases and then increases with the decrease of pH value in the acid solution. The liquid limit or plastic limit decreases and then increases, and the plasticity index exhibits change of fold line with the increase of pH value when soaked in the alkaline solution. It is shown that the influencing factors of the traditional diffusion layer thickness and the electric double layer theory are not applicable to the analysis of the consistency limits of Guilin red clay with different pH values. It is assumed that the liquid or plastic limit of the same soil has the determined potential. The qualitative analysis method on the consistency of red clay is proposed based on the double layer model. The formula for calculating the diffusion layer thickness of the red clay in Guilin soaked in the acid and alkali solutions is derived by considering the effects of various components of solution based on the traditional double layer theory. The new formula can make up for the deficiency that the conventional one is only applicable to the problem of the constant charge clay in the action of the symmetric electrolyte solution.