Based on the parabolic distribution function of ink pressure between two contact printing rollers, using permeating depth formula of printing ink in porous papers, which vary with the printing pressure and printing velocity, the formula of capillary radius was established. By measuring printing pressure, printing velocity and the permeating depth of printing ink in porous papers, the measurement of paper capillary radius was realized. The distribution of ink pressure obtained by this method is much more close to the practical distribution of ink pressure between two contact printing rollers, and the calculation precision of printing ink permeating depth in porous papers was improved. This new method is useful for studying infiltration of printing ink and the influence of capillary radius on printing quality.