Overland flow is influenced by the spatial variability of the watershed surface and the distribution of vegetation in the process of confluence. Thus, Manning's roughness coefficient, in different directions on the slope, has different values. This causes different effects on the resistance to flow in the downstream direction of each grid cell, affecting the flow distribution among the grid cells of a distributed hydrological model. To show that the spatial variation of the overland vegetation had the effect of directional difference resistance to the overland flow, this study used an indoor fixed-bed test. We used a cylinder to simulate the stems of the vegetation used in the study. We modeled the relationship between Manning's roughness coefficient and flow depth and studied this relationship for three types of vegetation distributed at three different slopes of 0.0%, 0.5%, and 1.0%. The slopes were based on three angles of 30°, 45°, and 90° between the vegetation rows and flow. The results showed that the resistance of overland flow had directional differences caused by the spatial variability of the vegetation distribution. At the same slope and flow depth, Manning's roughness coefficient decreased as the angle between flow and vegetation rows increased. At the same slope, the angle between flow and vegetation rows and Manning's roughness coefficient increased as flow depth increased. The slope did not affect the law of Manning's roughness coefficient with changes in the angle between flow and vegetation rows.