A transient shrinkage model was proposed for bio-porous material on the base of the relationship between the shrinking volume and average moisture content of particle. The changes of boundary radius, shrinkage displacement and the tissue shrinkage of every element layer of the pea at 40℃ were calculated. As suggested in the result, the external of the pea firstly appeared contraction due to water loss, rather than synchronous contraction from the outside to the inside with the wet sub-external migration and humidity decrease during the drying process for the pea. There is no shrinkage of parts of no loss water. Furthermore, the external shrinked more greatly than the internal due to more water loss in the external of the pea.