The shore-shallow lacustrine beach-bar sandbodies are widely developed during the period of the second member of the Shahejie formation of Paleogene in Chexi sag. The characteristics of grain sizes and their hydrodynamic meaning of beach-bar sandbodies are studied based on the description of cores and other correlated analyses and tests. There mainly occurs four kinds of grain size probability curves, which are low cutoff point-high slope-one bouncing population-one suspension population pattern, high cutoff point-high slope-one bouncing population-one suspension population pattern, two bouncing populations-one suspension population pattern and rolling population-bouncing population-suspension population pattern. Three kinds of grain size-standard deviation diagrams occur. The environmentally sensitive grain size components of beach-bar sandbodies has been studied using the method of grain size-standard deviation combined with the analysis of grain size probability curves, and there are four kinds of environmentally sensitive grain size components that are determined in the study area, which respectively represent suspension transportation, coastal current, wave and flow of distributary channel of delta. The wave and coastal current are the main hydrodynamics of the four kinds of hydrodynamics which control the formation and development of beach-bar sandbodies. The wave is the dominant hydrodynamic that controls the formation and development of beach-bar sandbodies of the second member of Shahejie formation in Chexi sag according to the calculation of the percentage of environmentally sensitive grain size components of wave and coastal current in the grain population.The percentage of wave environmentally sensitive grain size components is generally more than 60%. The effect degree of coastal current and its percentage in the grain population is very low. The percentage of coastal current is generally below 20% in Chexi sag.