Abstract: 【Objective】 This project aims to study the mechanism of cotton leaf abscission after TDZ (Thidiazuron, TDZ) treatment.【Method】Five cotton varieties (Gossypium spp) were used as materials. In the cotton base of Shawan, Xinjiang Agricultural University in Tacheng and the experimental field of Kuitun Agricultural Institute of Agricultural Division 7 in Tacheng in 2017 and 2018, two treatments T1 (4 days before TDZ application in the field) and T2 (the same time as the treatment time in the field) were set during the boll opening period. After TDZ treatment, the defoliation rate was investigated and five materials were sorted according to it. The activities of polygalacturonase (PG) and cellulase were measured at 0 h, 6 h, 18 h, 42 h, 66 h and 96 h, and the rates of leaf abscission, bulling and cotton yield were also counted.【Result】The results showed that after spraying TDZ, the cotton yield of T1 was significantly lower than that of the CK and T2 (P < 0.05); the shedding rate and flocculation rate of cotton materials by the two treatments were significantly higher than that of the CK (P < 0.01); the PG activity of leaf receptacle was significantly higher than that of the CK (P < 0.05) at 6 h, 18 h, 42 h, 66 h and 96 h; and the cellulase activity was significantly higher than that of the CK (P < 0.05) at 6 h, 66 h and 96 h (P < 0.05), and at 18 h and 42 h, it was extremely higher that of the CK (P < 0.01). 【Conclusion】The results showed that CIRI 35 and Xinluzao 50 were most sensitive to TDZ. TDZ treatment could regulate the activity of PG and cellulase in the exfoliated leaves, improve the rate of leaf abscission, and facilitate the mechanical harvesting of cotton.