Abstract: Taking the Quaternary red earth profile in Xuancheng as an example, we studied the influence of different pretreatment methods on grain size distribution of the red earth in southern China, The results indicate that:(1) The southern red earth contain a little amount of organic matter, which will influence the grain size distribution, so it's necessary to use hydrogen peroxide (H₂O₂) to remove the organic matter during the pretreatment. (2) Due to the strong leaching, carbonate content in red earth is extremely low, which has no effect on the result of measurement. So it doesn't need to add hydrochloric acid (HCl) to remove carbonate. The input of HCl may lead to flocculation, which will make the results coarser. (3) Compared with sodium hexametaphosphate ((NaPO₃)₆), sodium hydroxide(NaOH) leads to an abnormal shape of grain size frequency curves, and the decrease in grain content of size <1 μm, so NaOH is not an appropriate dispersant agent for the red earth. (4) According to the experiment, we find the pretreatment method of (H₂O₂+(NaPO₃)₆) is the best. It is recommended using H₂O₂ to remove organic matter, and (NaPO₃)₆ as a dispersant, finally dispersing the sample by ultrasonic cleaning instrument.