Our predecessors thought that heilu soils in our country were formed under modern biological and climatic conditions. The author's researches on the heilu soils in the middle and east of Gansu Province show that heilu soils are not merely the outcomes of modern prairie environment, also historical natural products. There had been some very long periods of soil forming processes under prairie environment in the early and middle Holocene and as early as in the late Pleistocene, ancient soils formed and the profile had complete humus horizons and calcic horizons. In the late Holocene up to present, the covering strata formed on the ancient soils under modern soil forming environment, the soil elements were recomposed, translocated and illu viated again in the covering strata and ancient soil horizons of the whole profile. The continued loess accumulation at a rate of 0.1-0.2mm per year is of great significance to the formation of thick heilu soil peofiles, especially in the north and northwest of Loess Plateau. The most striling characteristic of heilu soil profiles is that they possess the covering strata and buried ancient soil hirizons.