To explore the role of organic matter in Se(Ⅳ) adsorption by acid Se-rich soil, the occurrence forms of soil Se, the adsorption and desorption characteristics of soil Se(Ⅳ), and the changes of soil groups before and after adsorption were studied in latosolic red soil from typical Se-rich areas in Guangxi. The results show that: 1) After the removal of organic matter, organic bound Se in soil is decreased significantly, and Fe and Mn bound Se is the main occurrence form of soil Se. 2) The adsorption experiment shows that the adsorption process of soil to Se(Ⅳ) is mainly the heterogeneous surface adsorption of the multi molecular layer, and the adsorption process is controlled by chemical reaction and chemical adsorption. After the removal of organic matter, the adsorption capacity and intensity of soil to Se(Ⅳ) are decreased significantly. 3) The desorption experiment shows that the adsorption is mainly specific adsorption which is difficult to desorb. After removing organic matter, the desorption amount and rate are decreased, and K_d value is decreased. 4) Fourier spectrum shows that the adsorption of Se(Ⅳ) on soil is related to ligand exchange and electrostatic attraction. The removal of organic matter reduces the number of organic functional groups, resulting in the reduction of adsorption. In conclusion, the existence of organic matter improves the adsorption capacity and intensity of Se(Ⅳ) on acid Se-rich soil, which is an important factor for the low Se availability of acid selenium-rich soil.