Abstract: In this paper, taking for example the paleoseismic investigation of the Laohushan segment of the Haiyuan fault, the authors analyze the constraints on the completeness of paleoseismic records. Two fault-perpendicular trenches at the Songshan trench site have revealed that nearly horizontally-bedded sedimentary layers with alternating colors and grain sizes are offset, rotated and sheared. Such deformations in multiple stratigraphic horizons demonstrate the occurrence of six paleoseismic events. ^14C dating of charcoal from the trenches determines that all the six events occurred during the past 3500-3900 years, of which the youngest four events took place, from the oldest to youngest, during 0-410a A.D., shortly after 890-100Ga A.D., during 1440-1640a A.D. and in 1990 when the Jingtai-Tianzhu Mw=5.8 earthquake took place. This record and other examples suggest that the structural environment with few or no depositional hiatuses and high rates of sedimentation are the best sites for trenching in a strike-slip fault.