Abstract: Through the grain size analysis of 16 sandstone samples from the Upper Triassic Erqiao Formation in Zhongping area of Guizhou Province, combined with the regional geological setting, the paper studies the sedimentary environment and hydrodynamic conditions during the formation of the sandstone. The analysis on the probability cumulative grain size curves, C-M diagram, grain size characteristic parameters and scatter diagram of structure parameters of the sandstone indicates that the probability cumulative grain size curves of the sandstone from Erqiao Formation show good normal distribution, and the frequency histograms mostly show single-peak curves. The normal probability cumulative curves are mainly multi-bouncing population-one suspension population, one bouncing population-one suspension population, and one bouncing population-one suspension population-transitional zone patterns, mainly composed of suspended and jumping components, but lack of rolling components. The Upper Triassic Erqiao Formation in Zhongping area from the lower to the upper part is in a sedimentary environment transitioning from delta plain and delta front facies to shallow lake facies.