Abstract: Due to the advantages of low cost, good seismic performance, simple construction, etc., reinforcement method has become a common method of foundation improvement. However, the geosynthetic used in the reinforcement method is not natural and environmentally friendly. To solve this problem, the natural coir fiber is used to reinforce the red clay foundation in this paper. The effect of the coir fiber reinforcement depth on the deformation and bearing capacity of the red clay foundation is studied through load tests. The distribution law of the earth pressure along depth is analyzed. The results show that the load bearing ratio increases significantly with the increasing reinforcement depth, but as the settlement to the foundation width ratio increases from 0.1 to 0.6, the load bearing ratio generally shows a trend of decreasing and then stabilizing, and the greater the depth of reinforcement, the larger of the reduction ratio. When the reinforcement depth is 2 cm, the earth pressures at different depths are significantly less than the applied loading. When the reinforcement depth is 4 cm, the earth pressure to the loading ratio at 5 cm is stable at about 0.6, and the earth pressure to the loading ratio at other depths tends to be stable when the loading to the maximum loading ratio reaches 0.3. When the reinforcement depth is 6 cm, the earth pressure at 5 cm is basically close to the loading, and the earth pressure to the loading ratio at other depths is basically stable, but they are all significantly smaller than the applied loading. Due to stress concentration, the earth pressure near or within the reinforcement range is close to the applied loading, and the earth pressure away from the reinforcement range is significantly reduced. When the buried depth of the earth pressure cell reaches about 2 times of the foundation width, the earth pressure to the load ratio is basically unchanged, indicating that the main influence depth of the loading is about 2 times of the foundation width. The conclusions can provide a certain reference for similar engineering.