Abstract: A small amount of organic complexed chromium compounds remain and then enter into the bio-treatment system after the pretreatment of chromium-containing tannery wastewater. Thus, it is of great significance to study the adsorption and de-complexing behavior of organic complexed chromium compounds by activated sludge. The bio-treatment results of actual wastewater show that the aerobic activated sludge could simultaneously adsorb chemical oxygen demand (COD) and Cr (III) within 2 h, and the removal rate of Cr(III) reached 92.5%, while the removal rate of COD was less than 15%. Before and after adsorption, the molecular particle size of organic complexed chromium in activated sludge was concentrated in the region of 2-3 nm and less than 2 nm, and a small amount of organic complexed chromium remained after adsorption. With the prolongation of biochemical treatment period, Cr (III) gradually entered to the interior of microorganisms. Cr(III) at low concentration could promote the secretion of extracellular polymeric substances (EPS) in microorganisms, while high concentration of Cr(III) could inhibit its secretion. Cr(III) was mostly located in tightly bound EPS and loosely bound, and functional groups such as -OH, C-O-C, C-OH, and -COOH on EPS could provide advantageous adsorption sites for Cr(III). This study provides a theoretical basis for the total control and advanced treatment of chromium-containing tannery wastewater.