In order to study the chatacteristics of different soil erosion and sediment yield indoor and outdoor under rainfall condition,the erosion processes of Mangshan mountain loess were tested under laboratory and undisturbed conditions on 25°steep slopes under artificial simulated rainfalls.The similarity characteristics of the indoor and outdoor process of erosion and sediment yield were analyzed by the fuzzy closeness degree calculation method.The results show that with the rains of same intensity,runoff amount,sediment concentration, shear stress,and sediment yield of different soil bulk densities were fairly low at the initial stage and increased gradually over time;when the rain intensity reached the hyperplastic spot,these indicators increased sharply to the steady state within a few minutes.On 25°steep slopes,the nearness degree of the shear strength,runoff rate,sediment concentration and sediment yield of both indoor and outdoor simulation were very similar for the two kinds of soil bulk densities.The nearness degree of the high bulk density(1.2 g/cm3)soil was greater than the low bulk density(1.0g/cm3)soil.This implies that in order to achieve a laboratory condition similar to the field condition,the soil with higher bulk density should be applied in the simulations.