Preferential flow is a rapid infiltration form of soil moisture which occurs widely in paddy fields. This paper concentrated on typical farmland of paddy fields with straw mulching and non-cover conditions in the Karst region of Guangxi, using the methods of brilliant blue dye and the morphological image analysis technology to present the soil preferential flow features of two tillage types. The results showed that under the same water supply condition, the soil dyeing morphology of straw mulching paddy fields was mainly uniformly distributed, while the non-mulching conditions paddy fields showed obvious branch-like dyeing morphological and throughout the soil space. The average dyed patch shape coefficient of non-mulching conditions paddy field was 13.96, and the dyeing morphology showed a significant irregular differentiation. The average distribution density of dyeing water in soil space was 0.117, which was larger than that straw mulching paddy field, showing a concentrated distribution condition of dyeing water. The soil infiltration capacity of straw mulching field was higher than that of non-cover conditions paddy field, with its the average total dyeing area ratio was 46.69%. There was significant difference between them (P<0.05). But the time rate of preferential flow of straw mulching paddy field was later than the non-mulching conditions paddy field, which the average matrix flow depth was 16.92 cm. The preferential flow fraction of straw mulching paddy field was 27.47% less than the straw mulching rice paddy (49.55%). There was significant difference between them (P<0.05). Straw mulching method can affect the process of soil water movement in the field. At the same time, its increase the amount of water infiltration in the field to a certain extent and reduce the occurrence of preferential flow. Compared with the non-cover conditions of paddy field, the straw mulching method also promote the maintenance of soil water in the paddy field, and reduce the loss of water and fertilizer in the field.