To acquire planetary soil samples and return the samples to the Earth is an effective solution to understand the geological evolution of early stars. Drilling and coring by only utilizing the compound motions of rotation and penetration can sample subsurface soils relatively efficiently compared to excavating, melting, or digging. To maintain the stratification of soil samples for further analysis, a novel drilling and coring method called flexible tube coring (FTC) is adopted for future lunar sampling missions. However, given uncertain physical properties of lunar soil, which may seriously affect the coring results, suitable drilling parameters should be adjusted online. Otherwise, only a small amount of soil would be acquired under unsuitable drilling parameters. To understand the soil flowing characteristics of FTC, this study proposes a soil flowing state monitoring method by employing an ultrasonic sensor in a flexible tube to acquire the coring height and ratio online. Orthogonal experiments under different drilling parameters in one typical lunar regolith simulant revealed that the coring results are determined by the penetration per revolution (PPR) index and that under suitable PPR values, the corresponding coring ratio will always be high and stable. (C) 2017 Elsevier B.V. All rights reserved.