Abstract: Soil porosity represents the void spaces of soil， together with moisture content， determines the air content of soil， then influences the soil processes and plant growth. Under soil cover with organic mulch the soil porosity can be regulated for improving soil physical properties. Through field experiment the changes of soil porosity under different thickness of organic mulch were investigated and the results showed that on soil surface layer from 0 to 5 cm soil depth the average soil porosity was decreased in comparison with a bare soil, while the covers increased soil porosity in soil layer from 5 to 20 cm soil depth.