Plant extraction occupies a vital status in the fine chemical industry, and steam explosion has been confirmed to be an effective pretreatment to enhance the extraction performance. In order to reveal the extraction enhancing mechanism of steam explosion, innovatively from the view of plant porous medium, we characterized porous properties of steam exploded Radix Astragali and established the correlation of porous properties and saponins extraction performance of Radix Astragali. The results indicated that average pore diameter was the most relevant to extraction parameters with R-square >0.99 among various porous properties, which can effectively characterize the extraction performance. Area percentage of middle and large pores (100-100,000 nm) of Radix Astragali was increased from 8.25% to 91.57% after steam explosion and the increase of these pores was the major factor for enhancing extraction performance. Based on the close correlation of extraction parameters and porous properties, the extraction mechanistic model of steam exploded Radix Astragali was proposed. Altered porous properties improved the solute-solvent accessibility and internal mass transfer in the extraction process. Such findings are anticipated to prompt the enhancing effects and widespread application of steam explosion technology in plant extraction, and further identify research directions that lead to more efficient extraction. (C) 2014 Elsevier Ltd. All rights reserved.