In order to further study soil erosion mechanism of sloping farmland in low mountain and hilly region of western Liaoning Province, we selected cinnamon soil and brown soil as research object, periodic variation of water and sediment in rill erosion was studied under three slopes (5°, 10° and 15°) and three rainfall intensities (40, 60 and 80 mm/h) by using artificial simulated rainfall system. The results showed that the erosion process of cinnamon soil and brown soil slope could be divided into three stages, which were the pre-rill erosion stage, the development of drop sill stage and the rapid development of rill erosion stage. At the surface erosion stage before rill erosion, the flow velocity of cinnamon soil and brown soil increased with the increasing of rainfall intensity under the same slope condition. However, the effect of slope on flow velocity was not obvious under the same rainfall intensity. At the rill erosion stage, the flow velocities of in and between rills of cinnamon soil and brown soil increased with the increasing of rainfall intensity under specific slope. The flow velocity between rills of cinnamon soil and brown soil increased with the increasing of slope gradient under specific rainfall intensity. At the rill erosion stage, the flow velocity followed the order of in rill > on slope > between rills. At the rapid development of rill erosion stage, the runoff of cinnamon soil and brown soil accounted for more than 80% of the total runoff, and the soil erosion accounted for more than 70% of the total erosion. The contribution of brown soil to total erosion was more stable, and rill erosion was more likely to occur. The erosion pattern of the whole rainfall was the transformation from surface erosion to rill erosion. Once slope surface erosion occurred, rill erosion contributed more to the total erosion.