This paper presents the results of the field water flushing tests into sinkhole, exploration, investigation and laboratory sediment percentage tests in loess. It aims to examine the three dimensional distribution, hydrographic characteristics and erosion characteristics of the large-scale natural pipe-cave network in loess. The findings are summarized as follows. At first, the erosion process of the natural pipe-cave system in loess was periodical. Secondly, each of the water flow curves in the pipe-cave system includes three different characteristic stages: (a) the amount of water flow kept increasing, (b) the amount of water flow became constant, and (c) the amount of water flow became reducing. During (a), the retardance effect of water flow always happened. During (b), the amount of water flow had a limited fluctuation And the seepage quantity increased as the injected water increased. Thirdly, the changes of sediment percentage during the three stages were analyzed from the sediment percentage-time curves. The erosion velocity shows a positive correlation with the flow quantity from the accumulative comparison of erosion quantity in the designed cases. Finally, the causes of all the processes, phenomena and results mentioned above are explained. The data obtained from this work provide a valuable base for the research on both the mechanism and the velocity of sub-erosion in loess.