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During intense rainfall or flood irrigation, air may be compressed below the wetting front in the unsaturated zone, which reduces infiltration and thereby leads to excessive runoff and soil erosion. However, estimation of the critical infiltration rate (I-crit) that leads to air compression remains difficult. In this study, the critical infiltration rate for air compression was investigated experimentally and analytically. During infiltration experiments, significant air compression was observed as the infiltration rate excessed some critical rate for the working sands. And this measured critical rate increased as particle size of the sand increased. An innovative analytical approach was then proposed to estimate the values of I-crit using the parameters of the sands. When taking the impedance of air into consideration, the estimated I-crit matched well with the measured I-crit in the experiments. Based on the analytical approach, an equation was proposed to predict I-crit for a porous medium. According to the equation, the value of I-crit was determined by the permeability and pore size distribution of the porous medium. It was found that I-crit increased with the permeability of the porous medium, and also increased as the pore size of the medium became more uniform.