In view of the weak research on the unbalanced sediment transport characteristics of rainfall-runoff erosion under the combined action of rainfall and bed roughness, the variation characteristics of rainfall-runoff sediment transport under different rainfall intensities and slope lengths were studied by using the indoor simulated rainfall test. The results showed that: (1) Under the condition that the slope was a bare slope and the slope was 10°, when the slope length increased from 1 m to 8 m, the sediment concentrations of runoff increased by 3.28~7.15 times under the same rainfall intensity. The runoff sediment concentrations increased with the increase of rainfall intensities. When the rainfall intensities increased from 40 mm/h to 120 mm/h, the runoff sediment concentrations increased by 1.41~2.97 times under the same slope length. (2) According to the existing formula of sediment carrying capacity of river channel, after analyzing the influence of rainfall, slope, slope length and other factors on runoff sediment carrying capacity, and based on the rainfall test data and collected test data, the formula of sediment carrying capacity under rainfall condition was obtained, which could be applied to the calculation of sediment carrying capacity of overland flow under rainfall condition. (3) The influence of rainfall intensity on restoring saturation coefficient was discussed, and the unbalanced sediment transport model was proposed and verified. The verification results showed that the model had a good simulation effect on the variation of rainfall runoff sediment concentration along the slope. These results could provide reference for predicting the variation of sediment concentrations along the slope runoff and improving the erosion mechanism of thin layer runoff.