The degradation dynamics of toxin protein released by transgenic Bt cotton in soil were studied by the ELISA method, using Bt transgenic cotton cultivar GK 12 and transgenic Bt plus CpTI gene cultivar CCRI 41 as materials, and their parents (Simian 3, CCRI 23, respectively) as controls. The results showed that the degradation dynamics of Bt insecticidal protein in GK 12 was the same as that in CCRI 41 in leaves buried in soil. The degradation period was six months, and the Bt insecticidal protein degraded rapidly after being buried into soil for one to three months, and slowly from the fourth to the sixth month, and it could not be detected after seven months. The dynamics of Bt insecticidal protein of GK 12 and CCRI 41 in the roots in different growing stages were almost the same: the contents of insecticidal protein reached the peak in May(715.55 ng•g-1 and 531.23 ng•g-1, respectively), but decreased rapidly from June, fell to 45.63 ng•g-1 and 42.33 ng•g-1 in September, and decreased continually from October to next March, could not be detected after next April. The degradation dynamics of Bt insecticidal protein in GK 12 and CCRI 41 in soil were almost the same: the Bt insecticidal protein could not be detected before sowing, began to increase after the seedling stage, and reached the peak (324.06 ng•g-1 and 355.50 ng•g-1, respectively) in the flowering stage, then gradually decreased from the boll stage, dropped to 50.25 ng•g-1 and 65.76 ng•g-1, respectively, in the harvest period. Within six months after cotton harvest, the insecticidal protein decreased rapidly, but Bt insecticidal proteins could be detected until the following year in April.