The invention provides a method for clay inter-particle pore equivalent pore size. The method comprises the steps of fitting to obtain a whole soil-water characteristic curve, selecting data with matric suction being 144000kPa to 0kPa, converting the matric suction into relative humidity (RH), converting mass water content into water-filled pore volume in soil mass per unit mass, calculating a Kelvin radius, then calculating an actual pore radius and the thickness of a water film, then calculating an average pore radius, calculating the equivalent capillary diameter of the soil mass, and finally calculating the maximum height of capillary rise. By taking the whole soil-water characteristic curve as a carrier, on the basis of existing research, through a large quantity of experiments and combining with the theory of statistics, the equivalent method of pore size of soil samples with different clay content is obtained by derivation, and a favorable approach is provided for the calculation of the maximum height of capillary rise.