Soil crusts hinder water infiltration rate, thus increasing runoff and triggering soil erosion.Purplish soil is one of the most important types of soil in China and readily erodible.To study how soil crust is formed helps us understand how soil erosion proceeds and construct a physical erosion model.In recent years, works have been reported on characteristics and models of soil erosion in the purplish soil region of China.However, st ill not much is known about format ion of soil crusts and its effect on soil erosion process in purplish soil.In order to understand development of soil crusts on the surface of purplish soil and acquire scientific data for erosion risk assessment, a group of experiments were conducted.In the laboratory, two treatments (uncovered and covered with nets) were designed for samples of purplish soil collected in Lizikou Catchment, Sichuan Province.They were subjected to simulated rainfall.Results show that the purplish soil developed a stable layer of crust, 7~8 mm thick in 30 minutes; mechanical illuviation is the main factor of soil crusting, which was weakened by raindrops that beat the surface soil stimulating formation of a thin compacted layer; distribution of soil pores affects water infiltration and shear strength, which was demonstrated in the covered treatment where the total porocity was low, but evenly distributed, while the water permeability was high, because no compacted layer was formed at the init ial stage of the rainfall, thus lowering shear strength.